I to have a gelding that is well into his 30's. His teeth are kept up, but he still never put the weight completely on like I like. So I have switched him to an alfalfa pellet, +/- his senior. As horses get older their gut is not able to digest the nutrients in the hay as needed even if their teeth are kept up. The intestinal lining is just worn out and were not designed to be working for 30 + years. Horses are now living longer because of the great medical care they are now recieving. Older horses need a high caloric intake almost like a foal. That is why when you feed senior you must feed it properly to work. I use Purina products and my gelding is 1000 lbs (on a good day, he's a little horse).

So now that he can't have regular hay I use hay pellets instead.
Purina Equine Senior says if fed with nothing else he would get 14.5 lbs of Senior per day, broke into 2 meals. But if you feed hay or hay pellets you need to decrease the senior 1 lb for every 2 lbs of hay/pellet fed. So I decrease his senior by 6 lbs.
But since I feed him pellets(or hay) his weight requires him to have 12 lbs of the roughage per day, this is how his meals break down.

Morning: 4 lbs senior and 6 lbs of hay pellets
Evening: 4 lbs senior and 6lbs of hay pellets

Since we (the vet and I ) changed his diet and taken him completely off hay he has put so much weight on I now have taken him off the senior and just give him the pellets. He doesn't look his age at all. The great thing with the senior and pellets is that everything is pre processed so really they just swallow the senior and pellets ( I soak mine so they are soft) This way their gut doesn't have to work as hard to get the needed nutrients since the gut is worn out anyway.

Another great and cheap source of pure fat is using 1-2 cups poured over the top of hay and or grain of veggie, corn oil. ( I would only use this if your horse has no feet problems)
Commercial products, my friend uses Purina's Amplify which is a top dressing crumble that goes on their food and is a pure fat source. She uses it on an appendix bred horse that runs barrels and is a hard keeper.

Hope this helps.