It's not that he is not serious about practice (in fact, complains about others who goof off). He is looking forward to college where (he thinks) everyone will be more serious about playing ball and getting better.
It's really hard to explain, but like I said, I went through it too. There is something about real competition that just brings up a different intensity level. Some people collapse under it and others rise to it.
I've told him to try to create a competition within himself to dial up the intensity. Like with hitting, create game situations in his head instead of just taking batting practice. But I was hoping that someone else may have had a similar problem and found a way to fix it.
I'll also tell him what you said about thinking of the practices as competition -- for playing time. While it is a team sport, he is also in competition with his teammates. I know that he is aware of that, but he may have to change the way he thinks about "practice".