D.D.
Don't stop breast feeding!
Food allergies happen ALL THE TIME - especially if you're drinking cow's milk with the growth hormone which has been proven to be very harmful to cildren. This also could be causing the rash. Mucuous in the stool suggests that the "Good bacteria" are dying in the gut and are being flushed out. This could also be caused by allergens and also by antibiotics in the milk you are drinking. I only use organic milk for my children because of the harm that is caused to their bodies through the other milk. As you know already, your milk is being passed to your son so you have to experiment with what you eat and drink and see how it affects him.
That means you should be consuming plenty of probiotics to replenish the good bacteria that has died out. That will transfer to your baby as well through the milk.
My sister has experienced food allergies in both her children, and when she avoided the offending food, the problem cleared up. I still don't have a clue why MDs NEVER seem to check for food allergies when there is a problem. Her baby cried throughout the night, had gas, spit up a ton. Before I discovered Naturopaths, my son's doctor was always telling me spitting up was normal even though he was spitting up almost everything he was eating!
Anyway, cut out the food for a total of 4 days (one food at a time). If it's an allergen, you will see a difference - maybe not as quickly in the rash, but you will see it in the digestion. First try cutting out milk for 4 days, then wheat, then soy. Those are the top 3. It turns out my nephew has dairy, egg white and soy allergies. You could also try switching to organic milk and see if that helps - of course, you can't eat any dairy products that aren't organic during that time - either because they will have antibiotics and growth hormones in them as well.
Now that my sister has cut out the allergens, no more ear infections for the older son, no more digestive problems for her baby. Her baby was having acid reflux - when she cut out high fat and dairy, he immediately got a lot better. Also, there are other things you can do to help baby's digestion. Super Supplements (no, I don't work there, but have been helped a GREAT deal) is also good at helping baby problems linked to digestion. Their staff is VERY well trained. There is gripe water to try, there are digestive enzymes you can mix with water, put on your nipple so when baby's suckling, baby gets some of it to aid with digestion.
Another MD is a possibility - one that works with gastrointestinal problems, or a pediatric naturopath. By the way, cortisone creme isn't good for anyone.
P.S. I agree with the lady who said that you better wait on the immunizations. NO child should EVER be immunized before age 6 months. The go to the Dr. Sears web site for more information on a revised immunization schedule. His is much more sane. This can cause a whole host of problems as well.