You are asking a question that has plagued scholars for centuries. Good on you for noticing, though. Most never really notice this detail as it is only recorded in Mark. There are a number of speculations out there. I don't think any rise to the level of even a hypothesis, though. They are speculations. The truth is that there is simply not enough evidence in the scripture to build an opinion that can actually be argued. Here are a number of the guesses (I am sure there are more than what I am listing):
1st speculation -- Mark's mom lives in Jerusalem (Acts reveals that) so he left the garden while Jesus was praying and went home to catch some sleep, but is wakened by the men going to get Jesus and jumps out of his bed, throws on either his version of pajamas or his bed sheet and rushes off to Gethsemane.
NOTE: the narrative in the gospels suggest the 11 apostles (Judas was with the men coming to arrest Jesus) were in Gethsemane with Jesus, and while they did fall asleep, none are recorded as going away to Mark's mom's house or to an Inn. Certainly none are recorded as having been stripping down for sleep. Because Peter is glad of a fire later that night, we know the night was chilly at least. So, wandering around unclad in the chill night seems odd, especially on a mountain exposed to the winds. Because of these details, some say this makes no sense.
2nd speculation -- it is James, John or one of the other apostles. This is difficult for the same reasons as above. Why were they leaving for Mark's house or stripping to sleep in the chill night?
3rd speculation -- an olive press worker may have been reporting to work either the night shift or preparing for the early morning. A Mikvah has been discovered on the Mount of Olives near Gethsemane. To make kosher olive oil, the workers had to use the Mikvah (ceremonial purification bath) to become ritually clean. In this process, they strip, ritually bathe, exit and cloth themselves again. This man may have been going through the purification ritual when the spectacle of the soldiers coming would have led his curiosity to get the better of him. This sounds good at first, but this was the Passover. I am hard pressed to see a Jewish young man failing to observe Passover and going to purify for work. I may be wrong on that (I'm not Jewish, so I may be ignorant here).
4th speculation -- another follower of Jesus was going through the Mikvah, as Jesus and the disciples had earlier, to be ritually clean upon the mount of olives. The priests meant to catch Jesus and the others at the Mikvah so they could arrest them when vulnerable and unarmed, but were late. This kid was also late and got caught in the middle of the ritual.
5th speculation -- John 18:5-6 says that Jesus says I AM and this knocks the soldiers back and to the ground with power from the might of God. The words “went backward” come from the Greek word aperchomai. In this case, the words depict the soldiers and temple police staggering and stumbling backward, as if some force has hit them and is pushing them backward. Under this speculation, the boy is a dead boy whose body was naked after being anointed with oils and spices for burial and wrapped in his burial shroud. The word used for the cloth is the same as the word used for the shroud that wraps Jesus. According to this speculation, the power Jesus uses when he pronounces the name of God given to Moses (I AM) not only knocks the officials back but raises a fresh burial from the dead in the cemetery down toward the base of the mountain. The resurrected boy follows Jesus, knowing he was brought back to take up his short life anew by Jesus.
Personal Observation Here: This one is intriguing to many and I can't help but think of echoes of the Gospel of Nicodemus. The Gospel of Nicodemus was cannon in some communities of the ancient church but was later excluded from the cannon. The book is in two parts: The Acts of Pilate and the Harrowing of Hell. The Acts of Pilate is a retelling of the passion story and is the book from which the Catholic station 6 of the cross comes. Veronica is only mentioned in the non-canonical Gospel of Nicodemus. It is also in this gospel that the Roman that stabs Jesus is given the name Longinus. So you can see how influential this Gospel of Nicodemus was, since Longinus and Veronica have statues in St. Peter's Basilica in Rome. In the second half, Jesus goes to Hell (see also Ephesians 4:7-10 and 1 Peter 3:18-22) to rescue the dead that died before redemption, and the testimony of this is given to the Sanhedrin by two men who were resurrected when Jesus was resurrected. This notion of resurrections surrounding the passion of Christ seems to link this particular speculation with the Gospel of Nicodemus---in my mind at least.
6th speculation -- the boy is the angel that is waiting at the tomb later. This one is really odd to me as the word describing the angel's dress at the tomb is a word that describes a rich robe, not a linen sheet. Also, why is a naked angel running around with a bed or burial sheet? This one makes little sense to me, but some find it plausible.
7th speculation -- some have speculated that the boy is a sort of creative add on or is merely there to illustrate how the terror and confusion were that night. Under this speculation, it is a scene used by Mark to convey the horror the disciples felt and how they were terrified and fled.
Comment: This makes no sense in the Jewish or early Christian tradition. There is a longstanding belief that God does not throw insignificant or false things in the text and I think it is safe to say that if Mark is using creative license here, then the Gospel's authority has to be called into question. If it is divinely inspired, then God cannot lie (Hebrews 6:18 states it is impossible for God to lie) and it happened. Therefore, if you believe Mark is divinely inspired, then this kid was literally there and ran off streaking into the night. Buy why include this? The belief that God in the form of the Holy Spirit puts things in the scriptures for a reason is longstanding and both a Jewish and Christian tradition. Unfortunately, you have to do your homework to understand why the Holy Spirit wants a particular, seemingly random, detail put into the text.
Example: God and Moses reset the calendar in Exodus 12, making Nisan the first Jewish month, but prior to that the Ark of Noah comes to rest on the 17th day of the 7th month (Nisan in the pre-Exodus calendar). Why is that important? Why did God want us to know that Noah and his family made landfall on Nisan 17th? Noah is delivered on Nisan 17th and Jesus is resurrected on Nisan 17th. Thus the deliverance of humanity is accomplished on the same day - one a foreshadowing of the other.
Many other examples exist of little details pointing to significant revelation, and, thus, there is a long tradition that holds that God drops nothing in the scripture just for grins. If this is true, then this naked man has meaning. Unfortunately it appears the meaning and the greater story behind what this kid was doing there was known to the first century Christians, but was not written down plainly when the gospels were finally put to paper...or papyrus...or parchment...whatever. It appears that the person's identity is then lost to us, but would have been known to first century Christians and they would have known to link him to someone else in the scriptures. We have lost the link, though.
So there you have seven speculations. I am sure there are more. As you can tell, none really have any scriptural support. Thus, I call them all speculations. There is no hard information onto which one may hang an argument. So we are left with a large question mark as to why God and Mark wanted this detail presented in the text.
