He doesn't look thin, but he does need topline muscle.
Keep in mind that TBs are NOT supposed to be "big" or "beefy." TBs are lean, mean, running machines. Think of a greyhound dog.
Sweet feed is definitely not good for a TB. Oats are okay when he's in heavy competition or training, but not needed for weight gain during rest or light work (2-3 days, an hour or less per day).
Sweet feed is full of starch and sugar. Those things are high energy. High energy increases metabolism. High metabolism burns more calories, which makes the horse need more food to keep weight on. It's a vicious cycle.
To put on weight, here's what I would feed him:
- Free choice hay, even if they're grass out
- 6 qts (2- 3qt feed scoops, about 4-5 lbs) of Alfalfa pellets
- 1 cup of stabilized rice bran or 1 cup of milled flax (good fat source)
- Source Focus WT (probiotic/supplement for weight gain)
- Select II or other similar vitamin supplement (for horses on no fortified feed, all I have tried worked well)
- IF he's getting worked hard, no more than 3 qts (3 lbs) of Oats, but they're really optional.
You can add some water or a splash of oil to make it all stick together. Water works well; that's what I use.
Alfalfa is high in amino acids, which help build quality muscle. It also has good nutrition and quality calories, without all the sugar and starch.
Once he's up to a good weight, you can cut out the Source Focus WT and cut the fat supplement in half (1/2 cup). You can also reduce the Alfalfa pellets to 3-4 qts if he gets too fat.
My Anglo Arabian gelding (TBxArab, 15.3h) was a "hard keeper" when we first got him. We poured all kinds of food in him trying to put on weight and keep it there. We tried supplements, feeds, you name it! What finally worked as taking him OFF all of the extras, and just giving him quality hay, hay pellets, vitamins, and some fat. NO GRAINS. He became easier to work with, less spooky on the trail, and just generally "happier." When he works on the lunge or in the round pen, his head is lower and more relaxed, and he still has plenty of energy and "pep" for everything we ask of him.
I do give him plain whole oats when he's working hard, but only 2-3 qts, and only on those days he works hard. If I give him Oats daily, he will become a bit more "hyper". I can see a difference. Over the winter, without any work, he got too fat on just 1.5 lbs (about 2 qts) of alfalfa pellets and 1 cup of flax! (1/2 cup of milled flax is 4 oz by weight) He's now almost as easy of a keeper as my halter bred appy/qh mare, lol. I've had to cut back on their hay and flax down to 1/2 cup (2 OZ!)...
Give the new diet 30 days before you pass judgement. Be sure he's got access to hay along with his grass, so he's always nibbling on something.
I know, it sounds backwards to take a horse OFF feed to put weight on, but just trust my on this. It worked for 5 different horses at the last boarding facility I was at, and helped many others stabilize their weight and improve their attitudes. ALL of the horses at the facility improved in some way.
Here, here's a picture of both horses when they first came to my house. They grazed the pasture down quick (it's only 0.7 acres), so their weight was all put on with hay, alfalfa pellets, and flax. They put on weight in less than 2 months, and then ended up too fat, even with the pretty cold winter we had.
My Anglo is the one on the right. His belly is hanging down a bit due to lack of work, but he was just as ribby as my mare on the left.
Here they are a couple of weeks ago, modeling some saddles (new one for my daughter). Granted, they have not had ANY work over the winter, and were total mud balls before I brought them up.
Hopefully I'll have better photos once their hair all sheds out, lol. They both look like a couple of mules above...
Anyhow, just try it! The diet I gave has PLENTY of calories for your boy, plus fat, so he definitely won't lose any weight. Plus you should save some money once he's at a good weight. He will be able to hold his weight on a lot less food than you might think possible