Hi! While a trip to the doctors is always a good idea when you are worried, my first thought is a dairy allergy. When you are breastfeeding, if you eat/drink any dairy, the proteins will get into your breastmilk and cause a rash on a sensitive baby. If I were you, I would try cutting out all dairy (milk, eggs, yogurt, cheese) for 2 weeks and see what happens. If he gets better, you could try adding things back - it may be that he's sensitive to milk, but not eggs, for example. It's not hard to cut out this stuff for a short time, and if it doesn't help, no harm done. But if it does help, this could make a huge difference to your baby.
Also, I have found pediatricians to be quite variable in how seriously they take this. My own ped, who I otherwise like, pretty much dismissed me when I asked if my son could have a dairy allergy at that age and had me try aveeno lotion and use All Free and Clear detergent. Neither of these things helped at all. Friends convinced me to give avoiding dairy a try, and it make a huge difference for him. My sister had a similar experience.
My experience was that I could eat cheese and and eggs, but I couldn't eat yogurt or drink a glass of milk. My sister couldn't have any of it without her little on breaking out. On the plus side, my little guy outgrew his allergy by the time he was 1, so now he can eat anything. So don't worry too much if it turns out he is allergic to milk - just avoid it for now, and wait and see.