Write a letter to him for to read each day that he is gone (put days of the week on the envelope.)
Find pictures of the two of you, or the family, and tell him how much & what he means to everyone (be specific! Dad's proud of you when..., I love it when you used to..., etc.)
Pack a pillow case that has messages written on it from each family member and friends.
Stuff little hearts/messages into his socks.
There's always small bottles of lotion, tooth paster, bug spray,sunscreen that you could add a funny quip to, also. (I'll miss you "bugging" me. Love, your sister) ("You are my sunshine!" Love Mom and Dad) (Can't wait to "squeeze" you when you get home" written on the toothpaste) Corny as they may be, I bet he'll love it.
Try finding the things he'll need and have fun with them.
Hope this helps! Glad you asked about this. It made me think of ideas for when my daughter goes off to camp next month!
Blessings,
M.