Okay I learned from my niece who is an emergency room RN that most diaper rashes are a yeast infection - you can use the cream a woman would use for a yeast infection - or you can use a cream for athlete's foot or Jock Itch. Check it out on the internet if you think I am crazy. Her 18 month son had one while I was visiting and due to his age she got a dr to prescribe one. You could possibly talk to a pharmacist for a recommendation too.
It also helps to let him run around without clothes for a bit too.