I'm with the others. The vet is your best bet, and the tilted head could defintitly indicate a tooth problem.
Is he turned out with other horses? He could have gotten kicked, banged is head on something, and somehow broken a tooth that has abscessed. Or if any of his teeth just need a good floating and cut his cheek and that has become infected. even a wolf tooth that the bit clanks on can make them sore.
Your vet should also check for an obstruction at the back of the mouth. He could have stick somehow lodged sidewaysit's not likely though, (yes, it has happened) and it will cut the soft tissues and cause infection and make chewing and swallowing hard to do. Sometime there can be little thorns or stickers in your hay, that can stick in the soft tissues and get infected. There is a plant back in AR that we always called "Cheet" and it has been known to cause problems in the horses mouths. It's the seed heads that have a sharp point on them at maturity. It's a weed that is hard to keep out of hay sometimes.
Also, it's even possible he's got strangles and his lymph nodes are irritating him when eating hay (think strep throat--you'd eat the grain but stop on the scratchy hay first, right?) The vet should check his glands and his temp and make sure there isn't any snotty discharge.
And if vet finds nothing wrong, could be he's just getting picky about the feed and likes grain better than hay. Grain should always be a supplement if hay isn't providing enough energy for weight maitenance. Cut back on the grain, esp the corn!!! And see if his appetite for hay doesn't come back.