Thanks guys for your comments.
Just to be clear - his parents have never tried to justify his behaviour by using his conditions. I only know about his diagnosis because it came up in an unrelated convo with them. And yes as
@Death2unicorns guessed, there are other major factors with trauma.
If he ever plays up when they're there eg going on other people's properties, they properly shout at him straight away and make him apologise. But he doesnt do it often when they're there because he's occupied. It's when he's alone (and I guess bored) that the problem occurs.
My feeling is that he's often just so much to deal with that they send him outside. Eg when they have bbqs, he'll just want to copy them and join in and poke at the fire. No matter how many times they shout at him to stop, he'll continue. To the point where they have to send him out of the garden.