I believe the extra work is something that they, themselves, have to realize will help them. You can ask, beg, demand all you want, but it has to come from them.
I get it. As a parent you believe in their abilities and want them to work harder at it. But they have their own agenda and list of things that they feel are important in their lives.
My son has also always went to team organized practices, etc... with no complaints. But the value of the extra work had to be demonstrated to him. That came about a year or so ago (7th or 8th grade) when he struggled in his at bats at a baseball tournament. Where did he go at that point? Straight to one of the cages at the tournament site. All of the many times that I had told him that he should put the extra work in finally hit home. He did not enjoy his poor at bats and realized he had to put more work in if he wanted more success.
I'm not suggesting that this will happen with your son, but at some point he will get the message. Whether he gives up or gets down to work will give you the answer that you're seeking.