Tips to help a horse gain weight?

Hey,

As some of the regulars know, my horse is kinda high-maintenance. 20 year old TB with a high work load, so that's a given.

In the summer I really had him in condition- His leg muscles were developed enough he looked like a leggy QH from behind, and we could gallop a half mile, do a steeplechase course, XC course, whatever and he'd recover in under 5 minutes. This was achieved by 6 days a week riding, and a pretty committed conditioning schedule.

However, when school started, I stopped riding for the most part- 2 times a week generally- and he lost a ton of condition. However, he's also lost weight, and I'm having a tough time getting it back on. He was never fat- His body condition score was a 5 to start with, but I'd like to see more flab going into winter, especially considering we dropped off his work load so dramatically. He still has a lot of the muscle, but has lost some of the cardiovascular condition- We think some of the issues with weight come from all that muscle, plus the colder weather. He has a blanket on, and although he wasn't due yet I dewormed him with Pyrantel Pamoate on the 21st. If that doesn't help in a month I'll try Ivermectin, but I don't think it's worms.

He is fed high-quality (We grow our own) Alfalfa-Grass hay, approximately half a bale a day. (Each bale weighs approximately 50-60 lbs) He gets 13 lbs of grain per day, split into two feedings. (ADM Patriot 12%) I add soybean oil to each feeding of grain liberally. He's on pasture during the day, although that's going to be useless once it snows. He doesn't eat all of his feed in one sitting- He tends to pick through hay and grain alike, but he eats it eventually. He's never been a food horse, so this is normal. We add some molasses to the grain to entice him to eat more of it, which works well. He doesn't like sugar.

So, any suggestions on how to get more calories into him? We've been feeding him low-starch feeds because he gets rather hot on sweet feeds and the like, but he's been more lethargic under saddle lately- Likely due to a loss in condition as much as anything- so for the sake of more calories I might introduce some in, slowly so as not to shock his system. If he doesn't start gaining weight, or continues to lose it after deworming with ivermectin, we're calling the vet in. He's old and we did a lot of travel this summer- I wouldn't rule out ulcers. He's probably at a body condition score of a 4 now- He's never been one to put on weight easily, unlike the obese warmbloods on the place, but I'd like to see him more at a 5-6-7 going into winter. It's not too cold yet, high 40s to high 30s, but still....