Aww, poor little fella!! I've always been prone to UTIs myself, have had them regularly ever since childhood, so I know what he's going through! Pop into the chemist and grab a pack of Ural sachets, when you're that age you'll ignore the foul taste because it's fizzy, and therefore fun
If you can get him to, also give cranberry juice a try - obviously make sure it's actual juice and not the cranberry-flavoured sugar in water that a lot of brands come out with... I think that Ocean Spray or whatever it's called brand is the one I used to drink?

Definitely keep up with as much water as his little belly can stomach, but I know in my case I always needed an extra little boost to neutralise the infection, and Ural and cranberry juice always worked a charm. Good luck!!