How much attention are you giving your other body parts... Back training? Legs etc.... The body growns on a whole both strength wise and size wise. Working your back hard is going to help with your forearm size/strength, grip strength and will ad some size and strength to your Biceps...
I have had the same problem with dislocated shoulders. I find that flat bench with any type of bar puts a lot of stress on the shoulder. I have since moved to dumbell presses with a slight incline up to a larger incline. Also, delcine works well. Maybe have him start with some push-ups to...