My wife dropped off my DS at the airport this morning, alot of hugs in the pics my wife sent me of her and my daughter with my DS. As far as the mailing address goes, the FTU designation should be their Max number, which will be 4 for our children. My DS heard originally what undflyboy posted, but after the letters my DS sent to his buddies at Max 3 came back undelivered, we found out different.
Best of luck to your son!
Even my son who was doing the "I'm tough and I don't need any letters from home" shtick told us how comforting it was to come back to his bunk after a long day of training and finding letters waiting for him... Even from Dad razzing him...