WestCoastPapa:
A lot of great comments above.
When my son was in JH he also talked a lot about college and stated that his goal was to go to a D-1 school and play ball. His mother and I in no way made any comment to dampen his enthusiasm or his resolve. The one thing we emphasized to him was that he needed good grades to do so, and he took that to heart and from that point on always got onto the Honor Roll all the way through HS . . .and to our surprise and delight, he did it without without his parents ever having to push in any way. He had a goal and was self driven to get what he wanted.
On the baseball side, his mother and I simply tried to make it possible for him to play at the highest level he could get into in order to challenge him that we could stand back and watch him to see if he really had the talent and excel. And I must stay, it was great fun to watch and not have to do anything but play catch with him often and do some live pitching for some batting cage work. It didn't take long to where old Dad didn't know much about baseball any more and so he relied more on his team coaches. Old Dad still took videos of him on cage work and at his games that he liked to look at to see afterwards to evaluate if he was doing things mechanically like he wanted to. So, in my son's case, the best thing I feel we did for him was to be sure his parents were out of the picture except for providing support for what he was trying to accomplish.
Though I could see he had a lot of athletic talent during JH and afterwards, I was never sure just how far he could go and every year as he moved up that ladder I was surprised and delighted for his accomplishments. And even now, this is still going on as he makes his way though Minor League pro-ball (post a high level D-1 college).
So, with this experience I'd say don't worry about the transition and be sure not to project your own expectations onto your son, but do give him all your support in whatever he decides to do and enjoy the ride for however long it'll last. And keep in mind that things change AND they very often grow up much faster that you realize.