A week out I would start out adding soaked beat pulp in his feed, encreasing it as the trip got closer to about 2 quarts dry (before you soak it a day. The fiber you give him thursday will be in his gut saturday and beat pulp soaks up a ton of waater and will give him a bit of reserve. No idea where you are riding but watch your pace and don't pass up an oportunity for some green grass. Nibbling all day is way better than starving all day then pigging out in the eveneng.
Although more concerned with endurance horses some good info in this article aabout what to feed before and during the trail. Read part one and part 2 of "Beating the metabolic pull." Beating the Metabolic Pull: part 1 - Susan Evans Garlinghouse