- And got the T-shirt
Check his penis carefully, but I think that's the less likely problem. You do need to see an exotic vet for either the stone or a UTI. Don't bother with a small animal vet. "Small animal" means dogs and cats, not exotic pets, and general vets aren't trained to deal with exotics.
You do need to get him in quickly. A stone can fall into the urethra and completely block the flow of urine, and that's a full-on medical emergency. Syringe him extra fluids in the meantime. Most pigs love pedialyte (the generic is just as good as the brand name stuff, and costs half as much).