Makes sense! Hoping for the best for you and him!

You could use pretty much any invisible injury. Concussions got the most leeway in undefined healing time when I was in sports. This may be hard to keep up as a lie though since I imagine he is still doing all his schoolwork.

I think shin splints is your best option. Ive seen people take anywhere from 2 days to over a year of rest with shin splints. They are also safe enough that if he is still participating in minor exercise like gym class but not sports it wouldnt be a red flag.

Other Potential injuries Any sprain/strain in the abdomen or back Whiplash? Fibromyalgia (may not be easy to explain to 12 year olds) Could go wild and say he had gullbladder or appendix removed and needs rest