I wouldn't worry about doing it gradually to split his meals. I would just go ahead and cut it in half, twice a day.
And for what its worth, my own opinion would be to loose the bran and some of the oats. Oats will not help with weight gain and between the two this is just a lot of un-balanced feed being added. I also did not see that you were feeding any kind of grain? I would get him on a quality low starch senior feed or complete feed and work in 3-6 quarts as well.
Continuing with the "for what its worth" three of my Thoroughbreds are eating 4 cups beet pulp, 4 cups alfalfa, a bit of oats and 3 Quarts of a low starch-high fat Tribute feed. (Split into AM and PM feeding) The fourth Thoroughbred, who is 20, picky, slow eating and hard to keep weight on eats triple this amount and is "hand grazed" as well as on free choice 2nd cut mostly alfalfa hay (his "grain" is fed 3-4 times daily).
You can also try Max E Glo rice bran, works well and a cup of corn oil.