So what? The Scripture tells us to examine ourselves to determine whether we are actually in the faith.
The point of the Scripture is that when a person receives Christ, he becomes a whole new creature in Christ. That new creature cannot be demon possessed. If the old man of the flesh was demon possessed, he is now dead. He died on the cross with Christ and did not rise from the dead with Christ, so, there is a grave somewhere in heaven where the old man is buried.
I am not saying the guy is not converted to Christ, and so if he is, then he needs to see that his old man of the flesh is dead in Christ, and his new man is free from any demon influence.
So, instead of trying to get a demon cast out of him (which incidentally there is no examples of believers having demons cast out of them after the day of Pentecost), which is impossible because you can't cast a demon out of a corpse, he needs to come to Christ as the new man and claim his freedom. He should tell God, "I am sorry for my unbelief, ask You to forgive me, and strengthen my faith in Your Word. I now exercise faith in the promise of your Word:
"So if the Son sets you free, you will be free indeed" (John 8:36), and I take my freedom now in the Name of Jesus".
He can now say to the devil, "I am a child of God, and you cannot come near me with your lies - so, on your bike right now!"
If you are going to continue to believe that you are demon-possessed,
are you going to get the corpse of your old man out of its coffin and try to cast demons out of it? I guess someone would try if they are a senseless pelican!