Sounds normal to me. It is so, so early in his rehab; I'd just be thrilled that he has full extension. My son had full extension as well before he could bend it all the way (ie, to have forearm pressing against bicep). I am no doctor, but I'd would guess his doctor's would say it is nearly impossible for the repaired ligament to snap. I think it is more about getting the blood flow to the new tissue...which is actually a tendon. That is what is amazing about the human body...with surgeries like TJ and ACL repairs, a tendon (which is like a rope) replaces a ligament (which is more like a rubber band) and with time and proper blood flow, that tissue becomes, in effect, a ligament. Again, I am no doctor, but I did have my ACL repaired years ago, and it is very similar. This process of the tendon becoming a ligament is one of the main reasons the rehab and throwing program is so long for TJ.
Relax, be patient and moderate your emotions. It is a long road back, and there will be ups and downs. Ours has been a roller coaster ride, and that seems to be the norm.