Feed program for a underweight rescue Arabian?

I have a seven year old Arabian gelding, he's a rescue horse that we got about a year ago and he is still a bit under weight. right now I am feeding him four 16oz scoops of Timothy, four 16oz scoops of Purina Mills Equine Senior, 2 16oz scoops oats, and 2 16oz scoops rice bran, plus a mineral supplement in three feedings a day. he also gets hay morning and evening and is on a very large pasture all day. I was wondering if anybody might have any input as to something I could add to his diet or take away from his diet or feed more or less of something, Thank you!