So on a holiday day when you had no plans to go out, and with no prompting from you, what would he choose to do?
Sensory play is one thing to look at, what sort of things does he seek or avoid? You might need a bit of trial and error here depending on what info you got from your old OT. He likes the trampoline? Try and add variation with a ball, or balls, on there with him. Get him a mini-tramp for inside and sing songs or recite maths facts as he jumps. There’s endless ideas but it’s a very personal thing to your ds.
Does he like music? Listening, making it?
You said he’s good at maths I think? Does he like maths games, physical or electronic? What about kid’s picture books that explain maths concepts? We’ve got Sir Cumference ones, how much is a million, 365 penguins, the lion’s share etc.
You mentioned he likes taking things apart (or is it just destroying things?). Can you get him some mini-engineer take-apart/build toys? What about sitting down with him with some real tools and taking apart a broken toy or radio or something?
if ‘play’ is not his thing, will he do household tasks with you like sorting laundry etc. things that do have a ‘point’? These sorts of things are key co-regulation activities if you do them in the right way.
I think the key is trying to leverage what he’s already interested in in a new, out of the box way, rather than trying to get him to do something he’s not interested in. Also starting small and building, even if it’s a minute of attention at a time. Then adding variation. If giving up was not an option, what would you do?