Depending upon how thin the horse is, how old, and other factors, I'm only offering you what basic info I can to help a thin horse who isn't eating.
It's good that he's drinking, however if his water intake seems excessive to you, that may be a sign of kidney or other organ failure which, if you're able to have a vet come out to do bloodwork, you'll know very quickly if that's the case, and the vet would likely recommend humane euthanasia.
If the horse has hay (I'm presuming it's grass hay) in front of him and has no appetite, you may be able to try giving him some senior feed soaked into a mash, add probiotics to it, and give him small feedings several times a day if possible, spacing out his feed requirement by what his ideal weight would be, I.e., a 1,000 pound older horse (without molars) generally needs 8-10 pounds of senior divided into 2-3 feedings. In his case, depending upon his ideal weight, and how underweight he is, you may need to introduce less with far more frequency initially, and hopefully he'll show some interest and appetite and work up to eating a normal amount.
If his organs are failing, it isn't generally a good idea to give them any alfalfa hay.
He'll also need a salt block, and if he's not had any access to salt it's best not to let him have 24/7 access right away to one.
If he starts rolling, kicking at or looking back at his belly, those are signs of colic, and something to watch for. Most vets are good about selling banamine paste for horse owners to keep on hand in case of colic, and it's easy to administer it.
One thing to keep in mind with horses in general, and especially any emaciated horses, is that they are typically best off without any drastic changes in feed (and routine too), so anything you introduce, if you can do so gradually and in small amounts to begin with.
I hope a vet can examine him and that the news will be promising.
Last edited by DRichmond; 06-23-2012 at 02:16 AM.