P.H.
It sounds like atopic dermatitis, a form of eczema. My son started showing symptoms at about 3 days old, it was on his cheeks, shoulders, chest at first. My gp/obgyn couldn't identify it, because it came and went, oozing and then drying up. So I took him to the dermatologist and she id'ed it immediately. If your dermatologist isn't sure, I'm puzzled as to tell you what else to do, other than to treat it as a dematitis of some type, whether it be contact dermatitis from the car seat or atopic.
bathing him frequently in lukewarm water and slathering him in a good hydrating lotion such as Eucerin or Aquiphor is great for atopic. I would use cortizone when the flare ups are bad, but not on a daily basis. Try looking at your laundry detergent, have you changed it? Does it have fragrences or dyes? This is important because the clothing, and bed sheets, towels and wash clothes are all contacting his skin.
Maybe try covering his carseat with a removable cover that you can wash in your hypoallergenic laundry soap.
Try to keep him cool in this heat because heat makes it worse.
Good luck and I hope some of this helps.