Thanks for all this information.
It sounds to me like your best approach is in line with what you usually do, and that you'd do best to link it to behaviours, rather than feelings. In other words, there's nothing wrong with him having interest in or feelings towards girls who aren't of an appropriate age for him to pursue or have intimate relationships with. The trouble exists when he engages in any actions regarding those feelings that aren't appropriate. Can you make a list with him -- I feel like with is always the way to go, because the conversations that happen when doing something like that can be an engaged way of learning -- of behaviours that aren't okay, with consequences that will happen if he engages in them?
I know that sounds a lot like what you've been doing, but I do think what you've been doing sounds like the way to go with him.
I also wonder if there's any way to get him connected (virtually for now, obviously) with peers, including peers who are girls, around his same age who also have developmental disabilities? Peer connection always does all of us good, whatever our development, but I also feel like that might be a way for him to perhaps experience the emotional connection he has with people too young for him who are at or around his same age. That might be something you can do by accessing some parent support forums for parents of DD teens and work together to create a space for?
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