As noted above, my son had this surgery in February. So far, the experience has been similar to your son's. He progressed well right from the beginning but don't be fooled, its still a long process. One thing that the doctor and trainers all emphasized is that there is no rushing the recovery. Its great that there is no pain but you have to reign in the urge to think all is well. When my son finally reached the point at 12 weeks where he was released to begin the throwing program, he was amazed at how "weak" he felt on those first few throws. Despite having no pain and doing the therapy, a certain amount of atrophy does happen and you need to listen to the doctors, come back gradually, slowly increasing the distance. My son just finished his 7th week of throwing. He is now up to 120', this week he has to do 3 sets of 25 throws, 3 days. He can see the strength returning and now understands that if he tried to throw hard right away because he felt good, he would have hurt himself. One other thing that our doctor did was prescribe physical therapy over the summer at a local sports therapy group.While my son had the routine for PT down, he feels that the physical therapists do a really good job of keeping him stretched and emphasizing proper form.They've slowly increased band tensions and the like as he has continued.
good luck with this. We are optimistic on our side, it sounds like you are heading down the same path.