I've had rescued horses come in as thin as Baxter and worse. One thing that seems to work for me is Purina Omolene 400 and Ultium Competition. The 400 is designed to be a complete feed so that if the hay you're getting isn't the best quality the horse still gets the nutrition he needs. Depending on the size of the horse, I decide what weight he SHOULD be and I start feeding for that weight. For instance, a 15 hand horse with large bone should weigh approximately 1100 lbs when fit in light work, so I find the weight on the bag's feeding instructions and give that many pounds of feed. So on the 400 bag it says I should be feeding between 13 and 15 lbs of feed per horse per day. I would break that up into at least 3 feedings, preferably 4 so that he's not fed too much at once. I then add 1/4 lb of the Ultium to the feed per feeding for 1 week. Then after 2 weeks I add 1/2 lb per feeding and stay there until the horse has reached what I consider his ideal weight. I also free feed grass hay through out, I just make sure the horse eats his bucket first, then he can have all the hay he wants. I also put 1 oz of fine white salt on his feed twice a day and give lots of fresh, clean water for him to drink.
I've had a lot of success with this feeding regimen and gotten some skinny skinny horses back up to weight and shine with it. I also routinely De-worm with Ivermectin twice, 1 time now and then 1 again in a week, and then one month later I use a combination de-wormer to make sure I get anything left by the Ivermectin. I also have teeth checked and floated if necessary, and if any bad ones have them pulled. Sometimes the vet will want to wait on teeth until the horse is in better condition, that's ok, you can soak this mixture until it's nice an soft with no problems.
Looking forward to seeing pics of Baxter when he's fat, sassy and shiney! I've attached a picture of one of my horses, we call him Round Boy because he plumped up so well on this feed. He was very thin when I got him.
He's still in real good flesh, but he's not this fat anymore, he got a little tooooo healthy on this feed so I cut the amount a bit.