How many pounds does your half a bale equal. Everyone's bales vary in size and weight. Is he fed his meals (both hay and grain) in a stall or in a field; any competition for his hay? During the winter months in particular, my horses have free access to hay 24/7. Hope you can get to the bottom of his weight loss.
Ditto this, how big are your bales? Is he in with other horses who may be bullying him and eating up his share? My squares are considered big at 75 lbs (though in winter I have them on 1400 lb round bales, free choice) and to be putting weight on a thin horse I'd be feeding more than 1/day. Just to make sure he had hay in front of him 24/7. I realize he may not need the alfa alfa in a normal situation, I was just suggesting it to help put weight back on him. That's going to make the biggest difference, quality hay in front of him 24/7, and getting him into a stress free environment.
Even if he vet checked out fine 4 years go, that doesn't mean he's sound now. How is he moving? Navicular, absesses, etc...can all stress a horse out and make them drop weight as well. Cancer may be something to look into as well, though the horse isn't older he's right in his prime at 13. We had a horse get cancer in his sheath and he wasn't eating and dropped weight quite fast. You may want to talk to your vet about that.