Thrush will occur on the checks, not just around the anus. BBH fitteds don't have a wicking layer so could hold the wee to the skin for longer irritating issues even more. Watermellon is high in Phenol (carbolic acid) so could give him acidic/burny wee. I bet you'll find it's option 3 - diet related acidity. Have a smell of his breath. You'll be able to tell if it has a bile-like or sweet smell to it. Probiotics will help, so would drinking milk if he's not allergic, and of course cutting down on the phenol foods. Swallowing a teaspoon of apple cider vinegar a day is actually the best treatment but I don't know many kids who would be happy to take it (except my DD, but she's strange - she enjoys things like raw onion too).