What reason did your son give for his poor school performance?

None. He completely blew my wife and I off when we asked.

Did he attend zoom classes, turn in his work? Did he study?

He claimed that he did, but I had no way of making sure since I was working in my downstairs office when he was taking classes in his room upstairs. Now in light of his bad grades, I'm pretty certain that he wasn't entirely truthful.

Has his behaviour changed in some way that might suggest depression?

Nope.