This is what I have done with my son. He is one of those big dreamers. He is an 8th grader now but he has talked about nothing but being a big leader since he was seven.
I gave him the reality talk early on. I told him that the odds are remote that he would make it to the big leagues but that it is possible. I also informed him that making that dream come true would require a lot more work than he probably understood. I informed him that becoming a big leaguer would require practice and conditioning, training and a lot of off the field work for him to be a success.
I also told him that even with all this work there is no guarantee that he would make it all the way to the professional level. Illness, injury, something missing in their game or simply bad luck can derail that dream. I informed him that I was willing to do what I could (within my means) to help him reach that goal but I also needed a commitment from him to put forth the effort that this would require.
I also put a requirement on him that he needed to maintain his grades and remain a decent kid if I was going to put forth the sacrifices that I knew both myself and his mother would have to make. I informed him that even if he never played a game after high school or college there are still plenty of opportunities in baseball and maintaining his grades and acheiving something in the classroom would be the best way to succeed, both in and out of baseball.
So far he is still dedicated to his dream but there are times I have to kick him in the rear. He is a teenager now and sometimes teenagers act like, well teenagers. During those times when he doesn't want to go out and throw or do his band drills or some of his stretching and mobility exercises I ask him if he still has that dream. When he says yes I remind him that he needs to put in this work to become successful and if he doesn't want to do the work then I won't put in the effort that I do.
That works for him. I also keep everything open ended and inform him at least once every six months that if he decides that this isn't what he wants to do that it is ok, but he needs to tell me.