Is there a better workout than push-up to make a 11 yo boy to gain muscles (triceps, biceps, shoulders, back and chest) ?
He does not want to gain fat or big muscles, but just what he needs to play ice hockey and do hard shots.
When you are training to improve your sport performance, the exercises need to be matched with the movements you are doing in your sport. While push ups are a good basic exercise they are not ideal for the specific movements or explosiveness you need to improve your shot power in Hockey.
At age 11 you will gain the most improvement by listening to your hockey coach to perfect your technique. This does not mean you should not do other exercise but it will not give you as much improvement as better technique.