He sounds like the kind of horse who would bite off huge chunks of the salt blocks in this condition. This is a dangerous behavior as it can cause choking. Try himalayan salt licks, they're great. The horses can't bite off chunks and they don't dissolve in the rain. There's also another similar salt lick called "Redmond Rock Salt" that I have yet to try. ;) Himalayan Hanging Salt Lick | Dover Saddlery Redmond Rock Salt Lick | Dover Saddlery
Your horse should have salt available to him at all times, easily accessible by his water. He may consume the salt quickly, so make sure that you buy a couple at a time, so that you always have backup when he finishes one off.
When feeding, make sure you're feeding by weight, not by quantity. (Two pounds vs. 1 flake) Every bale of hay's flakes will be a different weight, and to make sure that your horse is consistently getting the portions he needs, it is vital to feed by weight instead of quantity. The same goes for grains and pelleted feeds.
It sounds like your horse could benefit from free-choice hay. It's not always a good idea to give free choice alfalfa, but a good option is to feed free choice orchard, bermuda, oat, etc., and small feedings of alfalfa in the morning and evening. In addition, feeding him a complete or supplemental pelleted feed would be a good idea, too. In most cases, hay alone does not provide a nutritionally sufficient diet.
Have you thought about supplements yet? They can be a great way to help balance and complete your horses nutritional needs. There are supplements for all sorts of things, but if your horse is an OTTB, there is a pretty high chance that he has gastric ulcers. Starting with a supplement like U-Gard to first heal, and then a supplement like U-7 Gastric Aid to maintain and prevent is a great idea. Digestion and weight gain supplements are also relevant in this situation, as well. If he's been nutritionally neglected, his hooves may be unhealthy, so a hoof supplement is also an option. Coat supplements may also be valid here.
I'm sure that you can't afford to give him all of these supplements, but in reality, you don't need to. Think about what the most important areas are for him. I would venture to guess that first U-Gard and later U-7 Gastric Aid are going to help him a lot.
For complete feeds, senior feeds are a good option for weight gain (and in some cases maintaining hard keepers). Depending on where you are at, Triple Crown and LMF feeds are definitely a favorite of mine.
Good luck, and keep us updated!! :)