@leviathan8382I like the loner angle too. And the voyeur ange, a lot, of course (being one myself).
He might be perceived as, well, different. Maybe a tad bit on asperger spectrum, meaning emotional intelligence doesn't come naturally for him, and he may react to touch differently than most.
For example in my teens, touching my ribs by a female hand was so sexually overloading I reacted with what most perceived as aversion. The easily out was claiming I'm ticklish. Also, highly functional sociopaths or light aspergers can appear inconsistent in social skills for normatives, freakingly wise one moment and hopelessly dumb another. Because, well, we do actually have to consciously analyze everything, lacking some functionality seemingly assumed to be inbuilt. It may lead to trust issues, the reactions and conclusions being different from expect, we are accused of lying when we're not, until we learn to tell expected lies.
So it may not be like he wasn't groomed at all at whatever age the family considered appropriate, it just went nowhere. Alternatively/additionally he might have been extra busy around the same time, with sports or other extracurricular out of house activities that put him in contact enough of strangers the family could see risk of doing any too rash, too obvious moves in fear of him talking aside, intentionally or not.
Then, it quite stuck at that. He started spying and voyeur, discovered what's happening. Meanwhile the family knew he's spying, but decided he's actually content with that and that's his role in the game now. They assume he knows they know and would come out and join if he felt the need. He may know they know, but still don't invite him, making him only more insulted, but may invent rationalizations like erroneously assuming he's still too young to be included. Then, those may be broken when, say, he sees other kids his age or own younger sister or brother included.
Each new level of the I know you know I know you know I know (ad infinitum) is exciting for the exhibitionist/voyeur game but is entrenching the roles and can be easily poisoned by miss-assumptions.