It's already been said, but I was going to suggest a "yeast" infection also. If absolutely nothing is working, there is a good chance that it is yeast, because they rarely go away on their own once they have reached open, weeping sore stage. My second son would get them all the time when he was little. Then my oldest would get them, and even my baby has had them. (is it something we are eating??) I didn't even know that it could be an issue with boys! On that note; be super vigilant about washing your hands when kids have these (I am sure you already are). They are easily passed from person to person. You might find yourself with an uncomfortable infection, otherwise. A candida diaper rash looks very distinctive, in that it appears as little pinprick type dots. If it gets real bad, the crease areas quite often turn VERY red and have a shiny appearance. You don't need to buy prescription cream, as long as it is for sure a yeast infection.
As playfelt mentioned, use clotrimazole from the drugstore pharmacy section. Or you can buy "Vagisil" type external cream. It's the same stuff, and yes, much much cheaper!! I have also been told to spray the diaper area with white vinegar, or wipe with cloth dipped in vinegar. The yeast cannot live in an acidic environment? I wouldn't want to do that on a bum with open sores, though. OUCH.

Thanks for all the awesome tips. Love the Milk of Magnesia idea. Going to suggest that one to the parents of my little p/t guy. He had a rash so bad a few weeks ago that he was walking bow legged, it hurt so bad. Poor dude. And my personal fave is Sudocrem also. Nice and thick, stays where it's supposed to. And it smells good, too! I would not do this at daycare, but a little naked time sometimes helps a rashy bum, too. Of course, once baby is up and moving, that becomes a risky little game.