Two's in is absolutely right Loopy Girl.
From my own memories, mail was power as a young apprentice. There was absolutely nothing better in the entire world, than being handed an envelope (or two) at mail time. Even though in these days, it will probably be far easier to ring him, believe me, he will appreciate a letter far more than a phonecall.
Also, try to encourage members of the family to write also. Personally speaking, I remember that during a particularly difficult time at Halton, I recieved a letter from one of my uncles who although very close to me, was not one for sharing his feelings. The letter he wrote me, explaining how proud he was of me, really did spur me on to greater things. Far greater than any phonecall would have done. The fact that I still own and read that letter to this day, is testament to the might of the pen!!
In all seriousness though, you should be very proud indeed, and proud for raising such a fine young chap. May I wish him happy times indeed during his basic!!!