Well, I guess the first thing to ask would be: what do you want him to do?
Is he supposed to wake up at 6, drink some raw eggs and hit the weight room? You say "motivation to work out on his own" but what does that mean exactly?
In any circumstance, I think you have to present the kid with options. If you think he would benefit from lifting, offer to drive him to the gym/ weight room at school/ etc. You could also offer to join a gym with him. Just a thought.
If you are referring to baseball workouts, again I dont think he is going to do it alone. Ask if he would be interested in attending any camps this summer. If he says no, that is fine. If he says yes, then try to find something that he could go to with some friends so that baseball is still something fun to do. Either way, you are enabling him to make his decision.
Even though there are some rarities, your son is not going to have a big league work ethic. He is a kid. Kids base their decisions on complete irrationality, so there is no way that he can see the same "big picture" that you can. All you can offer is support and opportunities.