Baking soda in the bath water is fantastic. Baking soda softens the water and it also has antibacterial properties.
With Michaela we would also make up a solution of baking soda and water to use in place of plain water on her bum cloths.
Don't use baby wipes, they're full of horrid things that will just irritate his bottom.
Another trick is to smear manuka honey on his bottom. I know it sounds odd but manuka honey is a well known wonder cure. Many international hospitals use manuka honey on their burns victims with great success (try and get one with a high UMF for best results and if you use cloth nappies you might want to buy some disposable liners while you use manuka honey).
When you see the doctor ask about Micreme H (has to be H and not just micreme, the steriod really does the trick to clear it up).