Make sure you get an address off him at every stage of his training and write to him every few days. Regardless of the content, there's nothing more uplifting than getting mail when you are in a strange place learning new stuff. (But don't write fluffy stuff on the envelope or he'll be ridiculed - not that he won't anyway in basic training, it's part of building them up!).
Send him the odd red cross parcel from time to time - CD's, chocolate, whatever he likes, again, always good for morale. Remember that he'll be putting in some busy little bear hours at first, so don't expect him to write or call as often as you would like, just be there when he needs you to be. It can seem lonely at first, but he will soon make new friends, and his age and maturity is on his side. It's going to be a big lifestyle change so be prepared for him to change - after his basic training he will be a a different person, but in a good way, more mature, confident and focussed.
What's he going to do? - it does make a difference to the training he will receive.