You do NOT need a vit/min supplement if you're feeding any kind of fortified feed, at the recommended levels.
3lbs of Ultium isn't enough. He's not getting the full spectrum and vitamins and minerals, so you're wasting your money. You need to either feed the recommended amount of Ultium (about 7lbs for a horse in moderate work) or stop feeding it and switch to Purina Enrich 32. Nature’s Essentials® Enrich™ 32 Supplement
Feed just 2 lbs of the Enrich, increase your beet pulp to 3 lbs dry weight, increase your alfalfa to 3 lbs dry weight, and add 1 lb of stabilized rice bran for added fat. Once he's at a good weight, slowly reduce beet pulp and rice bran. Cut it in half every 2 weeks until he holds a nice weight for you.
Alternately, you can feed just the beet pulp, alfalfa, and rice bran along with a vitamin supplement for horses not on fortified grain. My hard keeper (and TOO easy of a keeper) does better on that then on any grains/pelleted feeds. I'm using Uckele's Equi-Base Grass right now, and I have used Smart Pak's Smart Vite Maintenance Grass with good success. Just mix the supplement in with your soaked beet pulp and alfalfa. The Uckele supplement has high levels probiotics and Omega 3's in it. Both have higher values of nutrients than the Enrish 32.
Smart Pak: http://www.smartpakequine.com/produc...ctClassid=4579
Now, my horses keep a health weight on just hay, a little alfalfa, a little oats (just for fun), and their vitamins. You should be feeding as much GOOD quality grass or timothy hay as he will eat.