Could he have ulcers? Any horse on a high grain diet without hay has a tendency to get ulcers. The first sign is refusal to eat but without any signs of being in distress unless the ulcers are bad.
I just dealt with this with a friends horse. He went off his feed, wouldn't drink, was laying down but not distressed. The vet came out, listened to his vitals- all was normal. Tubed him full of fluid, Calcium, and oil. He came out of it fine. Started him on Ranitidine 3x daily.
What you are describing sounds very similar to ulcers. My mare went through a bout with ulcers as well. Her only sign at first was refusal to eat grain, but I didn't know what it was til she had a full blown episode of colic with fever. Turned out she had ulcers so bad she was anemic.
Only sign before that was a slight tendency not to want to finish her grain- she would nibble instead of eating at once.
I would go ahead and give him about 15 tums by syringe. Should see some response in the next hour or two. Tums is short term relief though- only lasts a few hours. You can get the human ranitidine at CVS pharmacy- the dosage is probably 10-15 pills for his size, but I'm not entirely certain about that.