My friend’s horse had a shoulder injury and because he was a young horse she planned to compete, she invested in having the vet do IRAP. It worked great and she got her horse back. But of course it requires stall rest and diligence in taking it easy when bringing the horse back. Given his age, I would seriously look into this procedure and or stem-cell. You can google IRAP for equines and it will give you a pretty good idea what it entails.
Hope this helps and good luck to you and your horse.