UserX
The reading I've been doing on Niacinamide suggests it's extremely beneficial for this.
You might want to find some way of giving him EFAs as well (Omega 3, 6, 9). Though this is difficult to do without mercury exposure.
Make sure he stays away from aluminum as well. If it's possible, maybe detoxing aluminum as well? Not too sure of the exact situation, but I suspect aluminum plays a huge role in alzheimer's.