diaper rub

My lo is 10 months and has had what we thght was diaper rash for about 3 months now ..the doc just said tht it's diaper rub causing it and we have tried butt balm,balmex,burts bee, and may others ..we also went up a size in diapers and let it air dry and still won't go away ..i even asked the doc if I shld switch to cloth and she said no ...Any suggestions plz !...