He'll be missing you, probably more than you're missing him. So personal/sentimental items are perfect and much more meaningful that sweets, mags, toiletries etc. It can also go a long way to sustaining your relationship while he's away.
Items such as naughty photos and worn underwear are ideal.
We run a free service whereby wives and girlfriends can send such items to us, we put them into a beautiful leather bound album complete with brass corners and a personalised name plate, and post them out to the correct BFPO. I've PM'd you the address to send them to.