UTI's can be very painful so I would at least call your vet & explain the situation. If it is a uti I'd get him to the vet asap. They are not usually life threatening especially if treated early (it could develope into a baldder infection, kidney infection & if left untreated kidney failure) but they do hurt like the high hevens.

Does he lift his leg?
You say it is the same spot, has anything changed (either physical or even just by smell) aruond that area lately?