Baby Rash

So my 6 week old baby keeps getting these rashes. I cannot for the life of me figure out what it’s from. I don’t use baby wash, lotion, or laundry soap that has fragrance in it. I showed his doctor and she said it looks like a heat rash but we’re inside out of the heat pretty much almost all day and he still has it. I was looking to maybe switching to formula if it’s something in my breast milk. Maybe he has a dairy problem. Any advice would be much appreciated and what’s the best dairy free formulas?