I have found the Burt's Bee's "Baby Bee" diaper ointment to be VERY good. That is what worked for my son.
Also, the "Florasone" ointment, which is a natural cortisone alternative to hydrocortisone creams. Whole Foods has it or Amazon.
OR, your son may have a Yeast infection rash. This is common. If so, you need to use a Yeast cream for it, because the usual diaper creams will NOT get rid of the rash.
If it is a Yeast infection rash.... then your Doctor should be able to diagnose that and tell you. You tried Lotramin, which is a Yeast cream. But there are various yeast creams with different active ingredients in it. Our Ped, suggests a yeast cream with "Clotrimazole" in it, for example.
Also, it could be something he is eating.
How OLD is your child????
If breastfeeding, it could be what you are eating, too.
If nothing works, tell your Pediatrician, take him back to the Doctor, and get him re-checked.
It really sounds like a Yeast infection rash.
You probably have to get a Prescription cream for it, Nystatin, or whatever your Doc, prescribes.
all the best,
Susan