Weight gain suggestions

Just a random thought here, I am no where near as experienced as Peggy in these things. But the website Peggy provided you, Equi-analytical labs, also provides a service where they can content test your grass/pasture, as well as your hay (They are the company that tests my hay as well). I know you said your barn has tons of grass so you don't feed hay, but it might be worth finding out what nutritional value the grass is actually providing, although you could only estimate at best how much he is actually consuming.

No, we do not feed baled hay. As a barn, we feed hay cubes....cuts down the waste to nothing (horses can't drop them from their mouth, doesn't get all over the barn aisle, etc). In addition I personally also feed him hay pellets. The grass is merely his long-stem munching time versus baled hay. But rest assured, he is getting 13 pounds of hay per day.

I could guess, but I would probably be wrong....sometimes he takes 4 or 5 bites at a time before swallowing, so I would be underestimating, if anything!

Honestly, I live in Florida, and we have sand for soil. I know that the grass isn't good for much more than munching on, unless you have a mini or a really hardy quarter horse.

OK Seminole is hands down a better product then Nutrena. High quality and more consistency in product.

I think you are on the right track now with adding some fat first

I understood that from what you said in this post:

I was just saying since he IS on turnout, knowing what he is getting out of the grass would resolve that unknown mathematical factor you mentioned in trying to make sure he is getting completely everything he needs. If you know what he is or isn't getting from the grass, it will give you a clearer picture of what he might be lacking in other areas to make up the difference. The testing is relatively inexpensive. I wasn't trying to be offensive or discount the point of what you ARE giving him, just trying to make a suggestion that hadn't been mentioned already, in how to help your horse gain some weight.

But, with the ration balancer, I know for a fact that he is getting everything he needs nutrition wise, at least when it comes to a vitamin and mineral balance. The protein and fat are lacking, the fat content because RB is meant for easy keepers, and the protein because he should be getting that from his hay as well. So I am about 95% sure if I add in a significant source of fat (I've gone back to looking at Cool Calories 100) that he should pick up weight with ease. If not, then I'll start evaluating the quality of all of the feed, since of course, balance and numbers wise, he is getting exactly what he needs.

My point in re-iterating the crappy grass is that I don't expect it to function as a source of nutrients, because he's getting everything he needs from controlled sources. I just like grass turn out because its natural, and he doesn't crib as long as he has grass to graze on. As long as I know that he is getting at least the recommended amount of everything he should be getting, then everything else is just icing on the cake.

You might also add a probio to his diet to aid with digestion as well

From experience with my TB I would suggest adding a stabilized rice bran, either pelleted or meal. Be sure to get an all natural formula. If is pretty much an awesome fat addition to a horse's diet and because you are already feeding alfalfa the balance is already there.
Before the summer my horse go one scoop of blue seal sentinel performance feed morning and night (the scoop is about a quart) however once the heat started and flies came out he rapidly lost weight.
In the summer he now gets garlic morning and night to ease the bugs, he gets more grain(obviously not for you) and he get his alfalfa morning and night. He began to gain weight with addition of the grain however his ribs did NOT disappear until I added the rice bran to his diet. Now he looks the best he has looked in the 4 years I have know him!
I'm not sure if you were able to get his weight up but if you are still having trouble maybe this is a good option for you. :)