Poor baby!!! Okay, the first goal is to get rid of the rash. You may want to go to the pediatrician just to make sure the rash is yeast and not bacterial. It could be either -- or it could have started out yeast and he could have gotten a secondary bacterial infection since the skin was open and diapers are pretty gross places. Plain old Desitin won't get rid of the rash if the rash is a yeast overgrowth. But I have a great solution I've used: Go to the feminine section of the HBC section at your local grocery store and pick up some miconozole (yeast infection medicine). Ask the pharmacy for a 20 dram vial and lid. At home, lay out a piece of wax paper and mix about half and half Desitin with miconozole. Now you have the yeast medicine mixed with the Desitin so it won't just soak into the diaper and go to waste. Put what you've mixed in the vial and use that every time you see his bottom. That should clear up the rash within about 3 days. If that doesn't work, he probably has a little staph infection -- ooooh, those hurt! You'll probably get a prescription for Bactroban for that. Just make sure they make it ointment form!
The Miralax is good. Ask the doctor about the amount to use. But you can use that everyday.
Poor kid. He's going to have to get the potty stuff worked out before he goes to preschool, though, so good luck! If you have any questions and want to private message me, feel free!
Good luck!