Get him off Omelene 500. NSC levels are unsafe. Switch to Ultium or similiar that you can get in your area. And measure in weight not volume. I've had huge luck with Blue Seal Sentinel Performance, which is similiar to Ultium.
Feed him as much hay as he'll eat. And observe his foraging habits. Does he graze all day? Pick here and there?
No oils- add a Omega 3 booster instead (flax).
BP is ok but I bet you could give him more. My hard keeper gets 3lbs (measured dry) 2x a day. (thats a "scoop" if our scoops are the same)
Consider Cool Calories - I'm not a fan of its content but it still does wonders for my skinny TBs.
Good luck. I have a 6yr old TB mare who is always lean. I've had good success with the Sentinel and BP, plenty of hay (she's not a good forager), as well as keeping her teeth in check every 6 months and keeping her mentally relaxed as well.
Check out SmartGut...did wonders for a couple of horses I have seen it on. Also, what kind of hay are you feeding? My TB cross that has issues does really well on alfalfa when I am trying to fatten him up a bit...
Rice bran, alfalfa, Nutrena Empower. The 3 best things for a hard keeper!
My mare eats Bluebonnet Xfactor (which has rice bran supplement in it), eats alfalfa (3 flakes/day), and gets 1 cup of Nutrena Empower in the AM/PM, and she looks fantastic. My trainer's older schoolies and a TB that can sometimes be a hard keeper are on the same Empower supplement and look amazing.
I have had several TBs that I have evented for years now and I cannot say enough good things about Ulitium! As well as flax seed, rice bran and alfalfa as well. They have all come around on those things with me.
Lots of hay too! As much as he will eat and not waste.
Ultium has lots of rice bran and is very easy to digest as well.
That being said-
Check his teeth!
Sometimes an equine dentist is much better than just your regular vet. (no offense to vets out there!)
Ulcers could be a factor but you might want to get a fecal count on him to double check for parasites before I would scope him. Its cheaper than having him scoped and he could need more than just a regular dewormer for now.
Try that out and I would definitely move to the Ultium vs the O 500! Read the labels, you want something that is high fat not high protein.
Also if you add some flax seed to that, I use only the ground flax seed or the oil not the whole flax seeds. They don't digest as well whole.
My Ottb mare was a hard keeper for years and always on the thin side.
For a few years we seem to have found a winning feeding program, as she is in good weight, shiny, and very well behaved under saddle:
-Free choice good quality hay. She's a small (15.1h) horse and she gets a minimum of 6 flakes in a hay net outside of her stall door, and about 4 flakes when she is turned out during the day. She cleans it all up.
- Triple Crown Senior: 1 pound am, 2 pounds pm
- U-Guard pellets to keep digestion problems or ulcers at bay.
I hope you'll find what works for your horse. It's frustrating when look thin no matter what you try!