How is the weather? If it's extremely cold outside I would put some luke warm water in his stall. I have issues with sand colic so whenever my horse gets a cow-patty type poop I will immediately sand clear. Check his temp and make sure he's acting right and then keep an eye on him. If the poop gets any type of watery consistency to it then call the vet because as Kevin pointed out, that's a pretty big warning sign.