The legal drinking age on the ship is 21, in the Bahamas (if you go ashore) it is 18 but
DCL will not allow "drunk minors" back on the ship.They have a club for the 18-20 year olds (Sorry, I'm drawing a blank on the name, but I think it's called "The Stack" on the Magic).
My daughter was 19 at the time of our cruise and she did come in the lounges with us for the comedic act, the 70's dance night, etc. I think if it's in a lounge where some type of entertainment takes place, he'll be fine in there with you.
If I were you I would call DCL and inform them of your son's condition. Who know's, maybe they will have some specific activities he may really enjoy that you are unaware of (i.e., behind the scenes tours). Disney is so great about wanting everyone to have a wonderful time!

Hope your cruise is great!
BTW, since he is 18 I'm sure there would be no issue with him going to the adult pool!