"He's good enough with out practice and that's good enough for him"
Your kids may not know what they need to know. However, I'd suggest that you telling him he'll have to work hard, train on and off the field, and prepare himself mentally in order to acheive his potential, may not be the most effective approach.
My son seemed to respond best when he was exposed to college and pro players and heard what they went through to get as good as they were. Also, I suggest he read things about some of his favorite players. Often reference will be made to how hard they worked, or what they had to overcome. In addition, my son took a big step forward in his baseball work ethic when he was in rehab. at 14 he had developed an imbalance between the front and back of his shoulder and while he was not injured, he was in a condition where he was susceptible to getting injured. We were fortunate enough to get him into a rehab location that works with a lot of the pro pitcher that train in the area and he was exposed to how much work they did to come back from injuries. Since then he has been very diligent in all aspects of his preparation.