I use a few different supplements and diets for my horses since they're all hot messes xD.

My older horse with just about no teeth, eats a mash of timothy hay cubes, a small amount of senior feed, probiotics and most importantly ManaPro Weight Builder. She's thirty years old and you couldn't even tell anymore. It is a senior weight building powder but I'm sure a half scoop would help a lot.

My arab has an annoyingly fast metabolism so I worked with my vet a little on his diet, and we cut his grain way down and added some soaked Timothy hay cubes to his food. Honestly he's a bit chunky now in the winter with less exercise 😂

Another one of mine has cushings so she gets her own special little diet.

All of my horses get pasture grass and hay while they're out. The biggest challenge for me to get everyone's weight under control was to remember that they need to eat 15-20lbs of that a day.

My vet definitely helped a lot too, couldn't have done it without her 😅

Any chance of ulcers? They could prevent him from gaining weight/maintaining weight.

I've had B and Tyerone on a few supplements for those. Purina Outlast and Gut X are probably the most cost effective (Gut X is pricey up front but a gallon of it last FOREVER). I have them on Platinum Performance GI too, but it's really expensive (however a lifesaver for Tyerone).

As far as weight gain supplements, I've used Purina Amplify years ago and really liked it.

2nd ulcers if the weight loss is very apparent and if he doesn't put on weight when his diet is adjusted for it. (since there's about a million ways to go about it. I myself really like ultra bloom.)

If it's just topline oftentimes building building muscle is the way to go, but I know that may not be an option for you right now. But trotting on the lunge line can be good.

My girls are looking a little odd themselves after having the winter off.

For wormers maybe my vet is old-school but he reccomends rotating wormers - again I know thats often not the popular opinion anymore.

If he hasn't been wormed recently I'd defo start with a wormer. I aim to always worm at least twice a year April ish time and october/Nov as they can eat alot of bot eggs from scratching each other in summer.

Does he eat well? Could his teeth need a small check up? Sometimes end of winter they can be a little meh but the good grass should come soon 😅

P.s it's so nice to see a real horse forum post for a change too 🥰

We always do fecal tests before worming. its easy and noninvasive and helps prevent you having to give unneeded meds. Plus, the vet can ID which worms to treat for and make it all that much more effective. It was never that expensive either.

for weight/muscle what is he doing for work (if any)? adding feed will help with fat/weight, but muscle requires some activity. Assuming hes not undersaddle (or even if he is), hand-walks are great, especially over mixed/undulating terrain. Stretches are good to help mobilize muscle groups too. (topline, underline, neck etc) there's tons of info out there on that. backing up slight inclines is great for hind end and topline too, walking over raised poles/logs etc. even if hes not broke at all you could add poles to his pen that he has to walk over for self exercise. ie near feed/water/shed etc