This is the start of a boy becomming a man of honor, be proud of him, I am. He is starting something that will change his life in so many ways for the better.
He will need many letters from home, especially during the basic training phase, make sure he gets at least one letter every day. Even if all you do is tell him your plans for the day, doing dishes, gardening, ect. Those little mundane things are what he will miss even if he does not know it yet. Handwrite them, NOT from the printer. He will want to know you handled it, took the time and was really thinking about him.
One of the very best things I ever did was join the USMC, and I hope your son feels the same about the Army.