I was usually okay on buses as a kid unless I tried to read. I think they are more bouncy than glidy, and that helps.
Try to emphasize to him that it’s better to be a little too cool than a little too warm. If he’s wearing a jacket, for example and taking it off would be a little cool but leaving it on a little warm, he should take it off. It can make a HUGE difference.
See if he can sit near the front of the bus. That really helped when I was on a bus tour of Israel. You might have to talk to somebody to make sure he can do this, as bus seats are sometimes assigned on school field trips. If you can get him assigned to the front row, it might save him the embarrassment of having to tell the other kids why he’s sitting there.