How do I support someone with ADHD?

My teacher has ADHD (I know because he told us). He is humorous, caring, and very strict on himself and his students. He is one of the best people I know. But lately, he often said he was uncomfortable, and canceled some appointments with his students. Saturday night we were supposed to have class on a previously given problem set, and he did go through a few of them with us, but then he said he couldn't do the problems anymore and ended the class early. Then he canceled Sunday's class and didn't show up on Wednesday's class. He must be suffering from really severe conditions because he is usually a very responsible person and holds himself up to a high standard. So I'm really worried about him.

Could anyone give me some idea of what he might be going through, just so I can better understand? My online searches about ADHD were supper unhelpful. How long do those bad days usually last? Does it have anything to do with the change of weather in autumn? And I know he sometimes has headaches. Is this a very common symptom?

I would really appreciate your responses. I can't imagine how hard it is for you guys, and you are so so brave and strong to be able to withstand this. Your struggles are valid, seen, and felt with the most tender sincerity. Give yourself space to rest because you deserve it.

Sending lots of hugs <3 and sleep well tonight!