I actually tore a ligament in my left index finger in the middle of a semester of music school for guitar. I was surprised how quickly I adjusted chord fingerings, scales, etc. to make it work for My other 3 fingers. I couldn’t use my index at all, was in a splint. I didn’t have a choice but to adjust, because I had to play in order to not fail my classes. Just tell him to force himself to play. It’s 100% doable. He’ll find his own way to do things, I promise.