Agree with the vet call. Sounds like he may have aspirated some of his grain. Is he blowing "junk" from his nose? My horse did this and it was very scary. He is fine but thought he was going to die at the time.
I would do what your vet recommends. We aren't there to evaluate your horse (of course neither is your vet) and I don't think anyone here can give you proper advice without examining your horse. Just my thoughts on it. I wouldn't want to tell you to do something that could harm your horse.
When our horses have had colic, they try to bite at their sides. They don't want you to touch their side. They have no bowel tones. They do tend to lay down and refuse to eat or drink. That is my experience.