Not gonna lie....he has to keep practicing.
tell him to work on his pullups, shuttle runs, and pushups. whenever you see him tell him to get down and give you ___ (#) pushups and vary the pushups (close hand, open, wide, etc) if you guys do this, ill guarantee you he'll improve his upper body, plus he'll get a little used to being pushed around and being told to do pushups. my friend and i do that to each other all the time, its fun. It's an excuse to do pushups in the middle of the sidewalk.
another thing that i do, whenever im doing homework or just watching tv,i put a backpack with dumbells (sometimes with my calc and physics book (yeah they're that heavy! haha)) on my back and drop down and do pushups until i fall. trust me, once you start doing pushups without the backpack they become a lot easier.