They may have just been playing and you walked in at the wrong time, but if you think it was purposefully, I recommend talking to your 6 year old. Tell them that you shouldn't do that kind of stuff. If he doesn't listen threaten to not let them play anymore.

As a parent you need to make sure the behavior isn't predatory, where one child is forcing the other child to do something. Have you talked to the other child's parent to find out if they have observed this behavior in their son before? What you want to be discover is if this behavior is a type of acting out - either from a child having a bad (i.e. sexual) experiecne with an adult that they are repeating with another child, or acting out as in the child is stressed and this is an expression of that stress. I suggest you talk to a child therapist to get professional guidance. If one or both of the boys are homosexual you don't want to freak out and make them ashamed of who they are. (One of my dearest friends who is now 60 knew he was gay at age 5) But if homosexuality is not at issue then you need to pay attention to make sure the child is getting the help and protection they need. Again, seek the advice of a professional. Child psychology is a delicate matter and you need to make sure you are doing the best thing for both boys. Many people would not take the step you've taken in reaching out for answers. You've already done a good thing. I respect your courage. Now keep going and get the counsel of a professional.

It is normal and natural for young kids to be courious. they will soon go on to other things but may regress from time to time...if you make a big deal out of it they will do it more but will hide their actions better. Just leave them alone...