Time is key. He's going to have a harder time for a while no matter what you do.

Even if you think his sensory stuff is under control - look back at that. Drastic changes make these things more severe which means that they're less under control. Increase the sensory diet stuff if you can.

Increase the amount of other routine has helped for me. But I don't know how much that's because of my autism. It's worth trying though. Without the increase in routine I've been not being able to take care of myself as much, which means I'm having an even harder time with everything else. So more routine is better.

Of course there's transitioning him up to the older class rather than making it a sudden change - start with a partial day, then a full day, then part of the week, before it being all of the time there.

You can implement the simple reward system at home and see if they can at school, and see if that helps. Some of the students at the school I'm at (I'm a tutor) have that. For one of the students its if he can go say 30 minutes without kicking or hitting or screaming they get either some pretzels of 5 minutes of a board game or something else pre-determined by the teacher and the student. A timer goes on, if they do the behaviors they're trying to change, the timer restarts. After they are reliable at 30 minutes (5 times in a row without any restarts), it gets bumped to 45 minutes and slightly nicer rewards to choose from. Then 1 hour and nicer rewards again, and so on.