I truly doubt that there are many guys out there that would actually look at and go through a memory box to relive past memories. That is mainly female territory. A memory box is the equivalent of an emotional teddy bear. If you give that to him he'll probably put it somewhere and never look at it again. If you want to "be there" while you're gone take some pictures for him. The type will depend on your age and relationship level, but the main thing to remember is that men are generally more visual than women.
Its probably also important to ask what you hope to achieve by doing this. If you won't be able to see him much anymore then your relationship is probably over. If that is the case having reminders of you around is pointless since he'll eventually move on.
...one can be sure of nothing until it has already happened...