If he hasn't been before and doesn't have it, a copy of The Perfect Shot would be great. I read it and re read it several times before my first trip and it was instrumental in the success of my trip. It doesn't matter if you are an excellent shot if you don't know where to aim.
Of all the things I brought to Namibia my first time, the lip balm ended up very high on the list of things I found helpful. It is damn dry there. It won't be much of a gift, but I assure you he'll appreciate having it.
For something extravagant, I got my wife a pair of Swarovski EL binoculars before our trip. I have the bigger ones and got her the small ones. Mine were better when hunting from a stand, but hers absolutely ruled the day when we were walking. Not so much the weight as the size: they just don't get in the way like the full sized ones do.
It is cool there in the mornings. A fleece jacket or wool sweater is nice to have. Even if he is pretty well stocked on sweaters, a nice loden wool sweater is hard not to appreciate.
