I have a few things to add
1. Nothing is going to allow him to maintain all of his mass if he has prolonged limited use of an extremity.Even high dose tren won’t stop it (but would be less, but at what cost…)
The good news is that with rotator cuff surgery he won’t be doing any lifting for a while of that arm but it won’t be all that long before he is doing PT and moving it agaisnt gravity, using bands and such. That alone prevents significant atrophy compared to a non weight bearing situation
I had major reconstructive surgery on a knee (way more complicated than a ligament repair). I had to be strict non weight bearing for 6 weeks, not even allowed to take a step. That leg atrophied significantly, even running low dose 3 ad , normal dose 11-kt, moderate dose osta, and 20 mg mk-677.
I do think I healed faster doing so. I hopped around the gym and had people put weights on the bar for me , did a lot of hammer strength stuff, etc. I made significantly gains from a upper body physique standpoint during those 6 weeks while my leg withered away… but I don’t think it withered away as much as it would have
2. Start lifting the other upper extremity as much as possible . Definitely studies out there showing that working out isolate contra lateral side decreases atrophy. If having right shoulder surgery need be jumping on the hammer strength stuff and working out the other side. Reccomend focusing on 12-15 reps sets at 50-75 % volume compared to normal combined with plenty of protein in diet . Not the time to be in a caloric deficit
3. Depending on what trt clinic he uses he may have options. Some clinics do offer low dose nandrolone, GHRPs, bpc, etc