Here's the rice bran I use, with feed directions.This brand has balanced the calcium/phosphorus ratio.
Nutritional Information | Manna Pro
I don't live in an area where there is much calcium in the soil and alfalfa hay is hard to come by and expensive. I was giving my 16 hand horse 2 cups a day when I was fattening her up along with all her hay and grain. It's 18% fat so you probably won't need to bother with oil.
Assuming you are using a generic rice bran, it's high in phosphorus.
Alfalfa is high in calcium. They work well together.
Beet pulp is great for weight gain. Here is comes in dry shreds or pellets. Some brands have a little added molasses as a binder. It will swell up when you add water. Will swell roughly the same rate as rice we eat does when you cook it. Cold water takes a few hours to soak up and swell. Warm water is faster. Add enough water to make it the same consistency as oatmeal. Some folks like their oatmeal loose, some stiffer. Water is heavy. Remember that if you have a long trek to the barn.
Beet pulp is also high calcium.
Sugar isn't good for horses. They get plenty from the grass. They are designed to eat grass all day and night, not a big meal of concentrated energy. Nor will many tolerate the half fermented stuff that you can get away with feeding cattle. I actually rinse my beet pulp a bit to wash away some of the molasses.
As much as I like beet pulp I only feed it now when it's real miserable out. It's a hike to the barn and it's heavy all soaked. Still I like to give them a warm meal when it's storming. Makes me feel better at least and the horses seem to appreciate it.
Don't sweat the calcium/phosphorus ratio much. The gelding isn't growing or breeding. Excess will make for great tomato patch manure. As long as he has enough of both it won't be much of an issue. It's when you still have actively growing bones it plays havoc.
I know this doesn't answer how much of each. Play it by ear and work him up slowly. Grass hay you can feed all he can eat. I'd move up slowly with the alfalfa because it is more nutrient rich. Some horses are even actually allergic to it. I just use alfalfa pellets here. I add a few cups for each horse to their ration balancer.
You will find that most of the horse rescues use senior feeds with their skinny ones they take in whether they are aged or not. Easier for them to digest it after they haven't been fed right.
Generic store brand vegetable oil is usually soy oil. You can add a little if you want. Honestly it makes the feed pan messy and the rice bran will do the same thing. Another thing you have to work up to slowly. I added 1/4 cup to the feed when I first got my skinny mare but gave up shortly afterwards. She really didn't care for the greasy feeling and I didn't need another thing to wash frequently.
Hope I didn't confuse you more. Don't be afraid to ask stuff.
Would also love seeing pictures as your middle aged horse recovers.