See if you can see him when he's squeaking. If it's a UTI or a stone (more likely a stone), he'll raise his rear end slightly. You can also make sure he's on light colored bedding and watch to see if his urine is tinged pink, but even if it's not, that's no guarantee that he doesn't have a stone.
If it were my pig, I'd be calling tomorrow to get him in to a good exotic vet. Squeaking as you describe it is most often caused by a stone. But aside from being painful, stones can work their way down into the urethra (the opening from the bladder) to the outside and totally block the flow of urine. That's a medical emergency requiring immediate surgery, no matter what time of the day or night it is. Females can sometimes pass stones through the urethra, which is short and stretchier than a male's, but males almost never are able to pass them.
How old is he? And what exactly are you feeding him, and how much of it?
