I don't know @RoxieLoxy. My son had a great Y9 and fell off a cliff from the start of Y10. Apparently it's a thing, the pressure inevitably cranks up a notch. Ed Psych also told us that fear of the unknown yawning chasm of what comes after can be a big part of it, even when students don't express it.
Might an ed Psych assessment help? School might be able to give you some of their budgeted ed Psych time or you could look at commissioning one privately, or getting LA to fund it as input to an emergency AR process. (the last was beyond me when we were at this point, but you might do better.)
Ultimately though, you may feel out of your depth but there isn't anyone on the planet who knows him better than you do or cares more for him. Maybe this is stating the obvious but I wish someone had said It to me: it is your call if you think that being at school is counterproductive and making him ill. You'd need to involve the GP, sure, but don't wait on school to tell you that it is not in his interests to be there.
My son just "noped" learning at home, always has done really. I know it can work well for others, I just have no clue how to make it happen here.
I really feel for you. So much stress every day, for both of you.