Several logs of his preferred dip. Compression ziplock packing bags. Somewhat expensive, but noise cancelling headphones if he doesn’t already have a pair. A couple spare barber shaving brushes (for weapons maintenance in sandy environments)...they were always going “missing”.
With respect to the good suggestions on gold bond, cruex, moleskin, etc—I’d suggest half dozen of each in travel size, along with some other hygiene essentials: small toothbrush, toothpaste, hand sanitizer, etc...package em up in ziplocks. He can have one for himself, and several spares to hook his buddies up if/when someone forgets or supplies run low. Other: Deck of cards, pocket bible or book of religious preference, St. Michael the archangel medal/necklace, pre-addressed envelopes and pad of paper (digital age, email and all that—but holding a handwritten letter from him will mean a lot).
Laminated wallet photo of your family...can be tucked in his IBA or taped inside helmet.