I don't understand why you feel you need to forgive your son. If he was forcing himself on the other child that is one thing, but if this was curiosity between both children then there isn't a need to forgive. Kids experiment sometimes and need to be taught what is appropriate and what is not.
One day I walked into my son's bedroom and caught him and his little friend (who was a girl) acting inappropriately toward one another. I broke it up and talked to them both about was is right and wrong. When I took the girl home I told her mother about the incident. She talked to her daughter and I had a long chat that night with my son about what God thinks of such things. End of story. There was no need to talk about it again. The one thing that changed in the relationship with his friend is that they were only allowed to play in the room with the door open.
It can be embarrassing and our pride can take a blow. I remember thinking, "
My son did this?" But it is a childish act just like any other childish act and it needs to be viewed with the right perspective. Kids sin. How do you deal with any other sin your son commits?
Sister, I am not trying to belittle the situation. These are just my thoughts on what you shared. I will pray right now that you will have wisdom as you walk through this.