As everyone stated, have the horses teeth done. He has probably never had his teeth done since he wasn't taken care of, and cribbing or sucking wind on top of that his mouth could use some work. At age 5 a horse will have his full set of adult teeth. But they will be razor sharp, after shedding off all those baby caps. You would be surprised what a good float with power tools will do. Depending on how bad he has messed up his teeth from cribbing he should be floated every 1-2 years. If he cribbs bad it will be closer to yearly. The majority of horses (who don't crib) can go about every 2 years on floating as long as power tools are used. If your vet uses hand floats still it will have to be done yearly. Horses have to chew their food completely to get all of the nutrients out of it and if they have sharp teeth (which sounds like he does) they will begin to partly chew their hay and then swallow the rest whole. By doing this they begin losing weight no matter how much you feed them. He should start putting on the weight by leaps and bounds after the floating. You could always use a complete feed (Like equine senior, even though he is young) along with the hay. The great thing with the senior is it is calorie packed yet all they have to due is swallow it. I would stay away from the high carb feeds (sweat feed)But no matter what you choose to feed make sure you weigh your food. Horses should be fed by weight not volume. A horse should eat 1-2% of its ideal body weight a day just as maintance. 1-1.5% should be in roughage if you chose to feed a grain supplement also. He probably developed the cribbing due to the lack of food and bordom. It would be great if he would give it up but I would be surprise. He would benefit greatly also from a cribbing collar, THe best on the market is the leather miricale collar. When used properly it works great, The reason people say it doesn't work I have found is because they are not fitting properly to the horse. The electric collars work really well also.

Hope this helps