I've seen a post or two about the "shift star", it looks like a front sprocket with larger and fewer teeth, being a culprit for this. The only thing is, I'm not sure if that bike has one. Maybe every bike has one? I don't have that knowledge yet! Anyhoo, if the shift star hangs up before a complete gear change, I believe it can cause this. The threads I've seen were dealing with people grinding and reshaping the points on this star, to aid in tricky shifting. One thing to try, although unrelated, is to ask him to make a very deliberate shift when going from 2-3 and see if that helps any. Might not fix the problem, but if he is aware of what gear he is in, it may help with riding/racing for the time being.