Some good information in this thread: Does he look better?
When I owned my last full TB, he got 4 (heavy) flakes of Alfalfa hay a day, 2 flakes of Bermuda a day, a big 5-gal bucket full of soaked beet pulp, 1 lb of rice bran, and a vitamin supplement. NO GRAIN. He was 16.3h and kept his weight on beautifully, even when working hard (riding 5-6 days a week, 1-3 hours a day, jumping and trails). I lived in So California at the time, hence the mostly-alfalfa diet.
My last few half-TBs (here in Arkansas) have been on a similar diet: free choice bermuda/grass hay (at least 20-25 lbs a day, half of a small/light bale), 2-5 lbs of alfalfa pellets (less for maintenance, more for weight gain), 1/2 cup to 2 cup of fat (flax or rice bran), and a vitamin supplement, NO GRAIN.
Grain and many feeds tend to hype up TBs and other "hot" or "hard keeper" horses. This increases the metabolism, burns more energy, and makes them lose weight, even if you're giving them a TON of food. Taking them OFF grain and most feeds will help return that metabolism to normal, making weight gain easier!
When buying alfalfa pellets or beet pulp, make sure there is NO molasses and no animal fat. When looking for a vit/min supplement, look for one that is "complete" with high levels of vitamin e, magnesium, and amino acids. I have used, successfully, GrandVite, Select II, SmartVite Maintenance Grass (from smartpak), Uckele's Equi Base Grass, and I'm currently using Nutri-Plus++ (most complete of all of them).
My horses have done very well on a grain-free diet. They are calmer, healthier, do not need ANY extra supplements (improved feet and skin/coat), my mare's PMS is barely noticable (she used to be a real WITCH!), and they hold their proper weight more easily. My current TBxArab was a hard keeper and now keeps his weight on nicely with just 2 lbs of alfalfa pellets and 1/2 cup of flax!