remember that just because he has Autism and ADHD doesn't mean he is exempt from family rules.
you may need a time out area (one of the bedrooms?) - not for punishment, but for genuine time out.
his first day will have been full on social and sensory overload for him.
A quiet room where nobody else is, where nobody else is talking or moving around, might be an ideal retreat.
Or, maybe, the garden, regardless of weather, where he can swing and jump and shout and shake things?
I have a 7 year old with the same diagnosis and God knows I don't cope very well sometimes, so here are some of my strategies for you.
1 - 2 - 3 - Magic! really is magic. Count out the undesirable behaviors (shouting, screaming, squeaking, shaking things, kicking, stamping)
Time out is golden and can completely swing his mood.
Long walks - again, if in a big empty field, are fantastic. All that space and not too much going on to stress him out! take a football and the other kids would love it too. Give him clear, VERY clear boundries about where he can go and explain that if he breaks that boundry rule, you will leave the field/park/whatever
not explaining too much. I know this runs contrary to instinct, but my Ds1 responds better if I don't stand there and give endless explanations about why his cup must not be on the floor, and how fast liquid flows, and what it will do to my flooring. I just say "Pick it up please. Now. I'm counting 1 .... 2..... 3....." Once he has DONE it, THEN I explain, and he knows that he will get his explanation quicker if he doesn't challenge me about everything.
If he's being a nuisance, boot him out to the garden for a bit. not as in force out, but just tell him he can have some outside play time, here, have a ball.