My DS recently had a UTI, although he had tonsilitis at the same time so it's kinda hard to tell what symptoms came from what.

It sounds like a UTI, which unless he is rolling around in pain or had a fever that is ridiculous, my gut feel is it can wait for the GP. But you have to trust your mummy instincts and if you think he looks super unwell then maybe an after hours clinic if there is one near you?

Good luck, and I hope he's much better very soon