My current horse was REALLY hard to keep weight on at your horse’s age, mainly due to nerves. I worked closely with my vets (husband and wife team) and we started with beet pulp, 1+ c oil and Pro Force Fiber (similar to Fuel but beet pulp based) fed to the label or slightly more. I tried Cool Calories, which is a coconut based supplement but it didn’t help much. I also supplemented his hay with alfalfa compressed bales. 1) more calories and 2) cations because he had very minor ulcers (treated for 6 weeks) and was just a hot mess. I moved and we eventually cut out the beet pulp and oil (there were oil marks all over his stall where he pressed his nose) and transitioned to Pro Force Fuel. I was very happy with how it helped him keep his weight. Also, he was in regular work, and until last year, I supplemented his winter life with alfalfa hay.
I also talked with nutritionists at both Purina and Cargill (Nutrena) about my specific horse and probably could have fed him Purina Ultium, since it was so similar to the ProForce lines in fat content and nutrition. I had him on supplemental alfalfa bales in the winter to maintain weight because he used to lose a lot despite being blanketed. At the new place, he was out 24/7 on his own anally-maintained pasture supplemented with local hay in the winter. I had Equi-Analytica analyze his pasture so I could balance his diet.
My horse needed a lot of grain and when I first moved, the old barn owner wanted to cut the grain back because he was so hot. My vet actually argued with her that “This horse needs calories. This horse needs grain” which uncharacteristically closed the mouth of the former BO.
We’re still there. At 13 years old (almost), he’s become a freakishly easy keeper. He only gets balancer - no grain - for the last maybe 3 years? I really work to keep his body score no higher than 6 because he can put on lbs!. He’s still sometimes hot, but life experience has helped him deal with the things he think will kill or maim him and trust me.
He’s been in constant work and I’ll add that correct work builds muscle, not feed.
Good luck with your guy!
ETA: PS, if you’re in prairie states you likely have access to good alfalfa bales from CO-NM. When I lived in NM, they were cheaper than good grass hay. Your local feed store likely has compressed bales. Check it out!
Also, try compressed bales rather then cubes or pellets. The latter contain mostly stem while you can see what you get (amount leaf) in a bale.