What Causes Stinging Pain While Urinating?

my five year old son, who is uncircumsized is experiencing frequent urination, complains of stinging when and after he pees, and the head of his penis is red, all the way around his urethra. His foreskin only retracts just under half way. Took him to the clinic last week, thought he maybe had a bladder infection, all came back fine was told to put some diaper cream on it, did that for a few days no improvement.