It sounds like he is getting burnt out. I work with kids and it is relatively common when they spend too much time on one activity because (like anything else) there are typically a lot of quicker, easier gains up front and then, making a little bit of improvement can require a lot of time and effort. Hopefully you guys can find some middle ground between him completely giving up the sport and being full bore.
Maybe it would be a good idea to take a "holiday" from playing and try to get into something else for a few months or establish a set amount of time per day/week to do drills.
Here is an overview on burnout in youth sports - it is definitely a thing:
Burnout In Kids Sports | Children's Hospital Colorado