skin rush in my my baby

my 20 months old son has a skin rush under an armpit that I don't know how to treat. The doctors said it was normal dry skin, very common among scandinavians, and that I shouldn't do anything, but it is getting worse and worse (and I couldn't trust doctors less!). I changed the soap I use for his clothes but nothing changed. I tried to apply a very natural cream I make, nothing. Colloidal Silver , aloe (the fresh juice from the plant), coconut oil, urine. It's just getting worse and worse. Do you have any suggestion?
thank you very much
sofia