Hi L.,
I would stay away from the powders while he is broke out. The powder will act as an abrasive while the skin is broken and make the irritation worse. Once the irritation is clear, it can be used to help absorb the sweat from the the skin to skin contact, but stay away from it for now. Have you ever got sand in your bathing suit? The powder is much smaller than sand, but will irritate the skin in the same way.
Start with a baking soda bath. Put about a 1/2 cup of baking soda in the baby bath tub and let him soak for as long as he will. You'll be surprised at how much the reddness lightens after the bath.
To clear up the irritation I would do the following. Do NOT use baby wipes to clean the irritated area, instead use a warm wash cloth and soft as you have on hand with a little bit of baby bath. Get lotrimin antifungal cream, it will be with the Athlete's Foot treatments. Apply a thin layer of lotrimin to all the irritated areas, then cover with desitin to keep the lotrimin in place. When skin rashes last for an extended amount of time they are usually yeast diaper rash. Without a prescription from the doctor or using the lotrimin, the rash will not clear up as you've experienced.
If that doesn't work I would resort to gentian violet, which can be purchased from the pharmacy without a prescription, but it very purple and stains terrible. It will work though. If you want more info on that let me know.
Best of luck,
Trudi