I'm glad that the rash is healing. I am very saddened to hear that he is back in school with a heavy backpack and a walker. This recovery does not have to be so painful, if you take it very slowly. I followed the mantras and didn't have the pain. If it were me, I would tell him he can stay home and heal for as long as he needs to...even if if takes a year. (which it did for me.) Lifting anything heavy really sets you back after this surgery.
I feel he will catch up in school as he is older. To me, his health and emotional state is far more important. I'm sorry if I am overstepping boundaries, but it doesn't have to be this way. He sounds very intelligent, and there is plenty of time. I think he'll get there faster if he returns home and rests, without outside pressure.
The emotional part of this recovery is intense. I cried buckets, and I had an easy and smooth recovery.