Just my experience here (I'm by no means a vet!):
In the event of colic, movement is the key. Movement for horses is very important to help move food along in the gut. So, in the event of something getting stuck - be it food or a nice, big gas bubble, walking helps move it along. Or, what I know someone else around here does that seems to work like a trick to get the bowels moving... she puts the horse on a trailer! Almost on cue, they start pooping!
If the horse is comfortable enough to have an appetite and eat grass, then I wouldn't think he's feeling too horrible, so I wouldn't worry too much about what he's walking on.