He's building up, and not resetting completely over the weekend. He's resetting enough to cope, but he's in a pattern where he's not managing to cope and stuff is getting worse within the day, and after school it gets somewhat better, but not enough for the next day.

Over the weekend he gets enough for Monday to be good, but he's still building up overload. As stuff is for him until you get stuff figured out for him (not necessarily in school), he'll just keep getting worse, because he's never managing to fully recover.

Some of us get overloaded enough that we take this path. I did this too long.

Good news for you is that figuring out how to help him now will have a huge effect on him. Getting him help now, which you ARE doing, is the right time to do it.

He sounds like he's at the right stage where he's not gotten so overloaded that it'll be harder to help him with the longer term build up. So, as you work on the things to help, you might get to see more extreme changes. Not right away, but as stuff gets working in his sensory system.

But, it sounds like he both is particularly prone to the overloading building up path, and isn't too far along it. The fact that he's not too far along it is good news for you.