Unidentified rash - eczema vs yeast?

My son has had this rash for the last 2 weeks. It started as a very small area on his side that looked like irritation from his diaper and has now spread

where you can see in this photo as well as along the creases on his butt cheeks and to his butt crack most recently. We have been to the pediatrician twice and he confidently said that it is not a yeast rash both times. He keeps prescribing hydrocortisone (first OTC and then a prescription strength) because he thinks it might be eczema, but the rash keeps getting progressively worse. Just wondering if anyone has had a similar experience and what you think the rash looks like. Also worth noting we did try lotrimin af for a few days and it didn’t seem to help either.