My Anglo gelding was similar. We fed him SO MUCH FOOD just to keep what little bit of weight on. What finally did it for him was to STOP feeding him so much "stuff". We put him on a natural, forage based diet and he ended up gaining too much weight on a LOT less food.
First important thing, hay, GOOD hay, and LOTS of it! We only buy fertilized, high protein bermuda hay. Give him as much as he will eat.
Next, a good vitamin supplement. Smart Pak's Smart Vite line is good. I'm using Uckele Equi-Base grass right now and it seems to be doing well too.
Last, some extra calories and fat. I use chopped pre-bloom alfalfa hay. You can buy it in compressed 50 lb bags. When Mark was thin, he got 4-5 lbs of this a day. I like to add to that some oats. When he was thin, he got 3 lbs of plain whole oats mixed. For a fat supplement, I used flax meal or whole flax seeds, whichever I could afford. They both seemed to work equally as well (1/2 cup of ground flax, 2 cups of whole flax).
To keep his weight on, Mark only gets about 1 lb of the Alfalfa, 1 lb of oats, and no more fat supplement. And he is a bit chubby right now! Lol
If he won't eat more hay, or if he's in a group turnout and will have to share, or if his teeth are starting to go (have your vet check) then I would add shredded beet pulp to his diet. Feed up to 8 lbs of dry weight split into two feedings (soak it for 30 minutes, pour off the excess water, then add your goodies). Mix into each feeding of beet pulp 2 lbs of alfalfa (pellets or chopped hay) and 1 lb of oat (whole if his teeth are good, crimped if they're not).
Feeding him Uckele's Equi-Base Grass, Beet Pulp, Alfalfa pellets/chopped hay, some oats, and a fat source is BETTER than a senior feed because you're feeding whole foods, more fiber, no preservatives, and no excess sugars. You're eliminating potentially harmful ingredients from his diet like molasses, corn, wheat, and grain sweepings. Once his metabolism levels back out, you'll find that he gains weight easier and keeps it on with less food.
I was skeptical at first myself. It seems contrary to what's "right," but the proof is in my horse! And ALL of the horses that were boarded with us at the time. The whole barn went on a forage based diet and all of the horses' improved in condition, health, and even attitude.
BEFORE (2 weeks after we got him, just after a bath)
AFTER (okay, a little too fat, lol, but look, a shiney winter
Just right! (no bath! His natural coat, both him and my mare who's in the lead)