There are a bunch of things to check: is the bar in the right place, is his upper back flat when the bar is placed or is it rounded over, is he jacking up his elbows at all during the lift, are his elbows just too high to begin with, are his elbows flared out to the sides instead of tucked in towards the body, is he pressing the bar up into his hands/wrists instead of pulling is down as he squats, etc. If none of those, have you tried a wider grip? If none of those, have you tried thumbs around the bar?