if they receive mail, im sure it would brighten his days if you sent him a care package! especially if he’s not allowed a lot of phone time (im assuming) he’ll probably be readjusting to that and feeling a bit bored. could send him a fun book/graphic novel, a snack or two, and maybe a fun crossword puzzle/sudoku book?
if he’s actually in basic training then you can’t do much other than send mail and wait. when my ex was in basic at USAFA i compiled messages from all of his friends and sent them in one letter. give him words of encouragement and let him know that if he wants to talk you’d be happy for a letter about his day/week