I peronsally LOVE alfalfa cubes to put on weight, but I understand that not all horse do well with alfalfa. My OTTB gets 2 buckets a day soaked.
I would start with just oil and see if that helps-it's cheap and easy.
Or you can try alfalfa pellets, hay strecher, or some other hay replacer. I personally love anything that's along the lines of hay because it's natural. I hate when people pump all this extra grain into their horses for weight gain. Now, every horse is different and some do well on grain, but personally would try other things first.
My vet is always amazed how my horse just gets unlimited hay and alfalfa cubes and is fat for a 23 year old OTTB. No grain for him!
When my TB turned 23 or so, he really started losing weight. He'd never been an easy keeper, but even with the good amount of grain he was getting, he couldn't maintain a good weight. I had some people tell me "He's just getting old, and that's what happens."
Fortunately, we didn't find anything wrong with him, and the vets recommendation was "Just feed him more!" Bloodwork came back fine, and we had his teeth done (he's on a strict 6 month schedule with the dentist). We took him off the corn/oats/balancer mix he was on and put him entirely on Triple Crown Senior with alfalfa cubes and he's been maintaining ever since. Whenever he looks a little light, we up the grain. Since TC Senior (and most senior feeds) are high in fiber and well-balanced, complete feeds, my old vet (and the new one) swear you can't feed too much to these old, hard-keepers. One vet even told me she knew of some people who just opened a bag and allowed the horses to eat free choice in order to maintain weight.
At 22, this weight loss could just be the result of a slowing digestive tract which isn't as efficient as it used to be. Definitely look into ulcers, teeth, and run bloodwork. But if he otherwise looks OK, just feed him more. His metabolism changed.
I also have a hard keeper. He is a 10 yo OTTB and looses weight every winter... without fail! I've tried beet pulp and rice bran with his regular pelleted feed but now am getting the best results from Alfalfa Hay and Dengi (sp) hay. The Dengi comes in a bag and looks like cut grass or hay. I add two hand fulls to each meal ( I dump it in right with his grain) I buy the high fiber stuff and it has a touch of molassas in it so he gobbles it right up. I also put more hay in his stall at night so he never has the chance to run out and have an empty stomach. I've heard of adding vegetable oil but would shy away from that route!