It’s a pic of his pee:
- And got the T-shirt
Guinea pig urine darkens when it's exposed to the air as it dries, so some color is nothing to worry about. The urine of young pigs dries darker and oranger than that of older pigs. And since it's hard to tell whether color is accurately reproduced in a photograph, there's no way to know whether that's oxidized urine or urine with a bit of blood in it.
The characteristic symptoms of stones are hunching and squeaking when peeing or pooping, particularly when pooping.
A UTI can also cause a bit of bleeding, but if the pig has a UTI, the urine will probably have a stronger odor than GP urine usually does.
Is he showing any other symptoms?