Rash? Eczema? Allergy?

This rash has happened once before and I’ve cleared it up with Hydrocortisone, now it’s back and worse then ever. This picture was after one application of the hydrocortisone ( I didn’t think to get a pic when it was at it worst, just wanted to help my son) but it was super red and almost looked inflamed. His doctor won’t see him until May 6th for his 4 month appointment so any advice is welcome. He is formula fed and we also tried an Aveeno oatmeal bath last night to see if that helped.