In addition to ur personal points... Remember to write anything that will encourage him to perform all his tasks with confidence coz the atmosphere there is higly frustrating for them... Write things to bring a sweet smile on his face that will help him forget all his worries for the moment.... Good luck for ur letter..:)
In basic training, he is pretty much isolated from society, and kept busy from dawn to dark, so he doesn't have much time to think about anything but his training. Keep it upbeat, and personal. "Love you and miss you; Looking forward to seeing you when you graduate; I got an A in my nuclear physics class," kind of stuff.
It can be lonely in boot camp. Write a little something every day. It helps break up the rigors of training and gives him something to look forward to everyday.
Write from the heart.
You can start with hello.