YEP... YOGURT!!
sounds gross hey!! But I had thrush myself once and my Nan told me to put yogurt there...i turned my nose up at the idea and a day later she knocked on my door with a tub of yogurt.... i only used it the once and it eased it off straight away!!!!
Also if he is big enough to eat it that is good too as it works through the digestive system and can help clear the urine of whatever might be causeing it....
You could always visit the DR and get them to advise a trush cream..... would be similar to nappy rash cream I would imagine!

Good luck and I hope it clears up!!
xxxooo