Don't buy him anything you think he could benefit from in boot camp because chances are he won't have time to use it or he won't be allowed to have it. Also, is he planning on making this a long term career or is he just getting college money and then getting out after his commitment?
If he's planning on making this a more permanent career, then maybe get some kind of cool keepsake with the USMC logo on it. Some engraving places will offer anything from flasks, cigar humidors, pocket watches, etc. It just depends on his tastes. These gifts are thoughtful and will remain stylish for a very long time.
If he is just doing this as a temporary solution then it could be anything like a gift certificate to his favorite restaurant to have his first meal when he finishes or a yearly membership to something like netflix to keep him occupied during his first year of service or when he isn't likely to be deployed.