8 month old breaking out in rashes

so my 8 month old has very sensitive skin. I am super careful as far as soap and lotion goes. He slobber A Lot so I'm constantly changing his bib. He started breaking out mainly around his neck. I took him in to see his pediatrician because we have a family member who had the strep. anyway, they checked my baby and told me it wasn't strep and that since he slobber and wet his clothes, that's why he has the rash. PLease somebody on the same boat? I'm concerned because his poor skin looks so bad.