Watch out for sensory issues. If the kid reacts to, I dunno, styrofoam like it's full of shards of broken glass and he can't bear to touch it, keep him away from styrofoam (there is no training this sort of thing away, although you could get a learned-helplessness oh-they're-torturing-me-again reaction if you tried too hard - and I'm given to understand that some treatment plans do aim at that, presumably because suffering in silence is less disruptive for caretakers than screaming in discomfort). Keeping away sensory aversives frees up brainpower to do other things, as well as being an important form of not-torturing-the-kid.

Treat stimmy behaviors like ordinary fidgeting. They are only weird relative to a framework that an autistic does not operate within. Specifically, they do not say anything in particular about whether he is paying attention, especially to a person who he wouldn't be likely to make eye contact with anyway.

Explain substeps of tasks that do not get understood promptly. (If "get a bowl of cereal" results in the kid staring blankly into space, try "get a bowl out of the cupboard and put it on the counter, open the cereal box and pour cereal into the bowl until the bowl is mostly full, close and put away the cereal box, get the milk out of the fridge and pour it into the bowl until it comes most of the way up to the line of the cereal, put the cap back on the milk and put it away, get a spoon from the drawer and eat the cereal with it". And if that doesn't work, maybe the kid doesn't want to get a bowl of cereal. I'd certainly look funny at anyone instructing me to get a bowl of cereal for no obvious reason or just to see if I could; maybe I don't want any and don't want to perform tricks for someone who is curious about whether I can do them.)