coach not pushing athletes enough? what to do?  

It might be helpful not to judge the coach at first glance. If your son is winning his contest, then it's a good indication that his coach is doing his job. But when he doesn't have any improvements, that would be the time to consider other options. I suggest you talk to your son's coach and tell him your concerns. It is not good to talk at the back of people. You wouldn't want other people to talk behind your back. I hope your son will continue to be better and be the best!