Reading and learning is important but take you son to a licensed physical therapist and get a professional plan in place.
My opinion for what it is worth is you should not play practice sets or even have intensive practice for a few weeks after a substantial injury. You go through physical therapy to stretch and strengthen. Then, you start playing again slowly - like every 2 or 3 days for 30 mins, than 1 hour, 1.5 hours 2 hours. Then you play practice sets and finally matches. But, my opinion is just my view of common sense - go see a doctor or physical therapy.
I rehab-ed with just physical therapy for about 8 weeks after minor knee surgery. Then 3 weeks of gradually increasing practice before I played a real match and continue the rehab exercises to this day - at least 3-4 times per week. It has been 6 months and knee is doing pretty well.