Hi. The biggest thing I worry about in a young pot-bellied pig is a gastrointestinal foreign body. The fact that he is not holding down water and is feeling poorly, makes me very concerned. Main rule outs for vomiting are constipation in an older pot-bellied pig and foreign body stuck in the stomach or intestines of younger pot-bellied pigs.
I recommend having him seen as soon as possible. X rays can determine if a GI foreign body is present. Even if it is being caused by constipation, they often need IV fluids and enemas to treat.
You can typically use one of the over the counter fleet enemas fairly safely in these guys. Unfortunately, many of them will need iv fluids to correct any underlying dehydration to really help with the constipation. Metamucil can be used, but since he is vomiting, I probably wouldn't try giving him anything by mouth. Again, I am very concerned about a possible blockage. You can try the pre-packaged fleet enemas twice daily, but get him to a vet as soon as possible.