why is it a problem? Most likely he has less active range of motion than other people but I dont think it would affect the gains much. Either just continue doing what you doing if its safe or reduce the range of motion a little bit so the scapula doesnt move as much, that just my 2 cents. When u think about it the scapula is supposed to move quite a lot, its just harder to isolate muscles if scapula moves a lot. I never think about scapula when doing those exercises you mentioned, the more you move around while doing tricep pushdowns for example, you are propably cheating little bit but the muscles are still being stimulated as long as the weight is being controlled.