Zandor, amateur opinions such as ours are pointless in a complex case like this.
I suggest that the key factor in the boy's behaviour is the highly sexualised 'training' given him by his older brother.
And, that the only way to look at it, is not from the perspective of scandalised christian morality, but by examining what damage, if any, there may be to his emotional maturity. (I assume the boy is now around 15-16).
Only a skillled, experienced psychiatrist who follows APA guidelines may be equipped to do that. If you live in the USA it should not be difficutl to find someone like that. Of course, if I've estimated his age correctly, the boy (young man) also surely has a say in what he wants to do.
There may also be a legal aspect. Does his behaviour infringe legal codes - in the USA his behaviour surely would do that, not to mention his "clients."
Putting a kid like this into gaol is surely not going to help him.
And finally, your comment on his mother's description of his infant behaviour,
when this particular child, M, was born, she took it very personally that the infant did not want her to hold him. She talked about how the infant would cry when she tried to hold him and wouldn't stop crying until she put him down, and how deeply hurt she felt that he wasn't like the older brother...
may mean that autism is a factor.
But only an experienced, professionally trained person can make the assessment. And for the boy's sake, try to steer them away from amateurs, as may find among the witnesses (and, other Chrisitians, who think that reading the Bible qualifies them to do anything) and encourage them to find professional help.