The substrate should not be sand unless you have it mixed with something else, and then only 20 percent of it should be sand. Sand causes impaction. What are your temps? I would give the sully a warm soak for about 1/2 an hour, and get rid of the sand or mix it with something like coconut coir or cypress mulch.
Is it foam or sand in his mouth? If he's foaming at the mouth, then thats a serious problem. He could be overheated, he could have ingested a toxin, or he could be vomiting. Have you considered taking him to a vet?