My son had this happened to him after a couple of weeks of starting him with rice cereal. It was an allergy to the formula I was mixing the cereal with.
The way I found out was because, he not only had the blotches in the facem but also in one of the arms he had the food on when he touched his face. I mixed a little bit o rice cereal with water and rubbed a dot on his tummy and nothing happened. Then I rubbed a little bit of formula too, and sure anough the spot got red a few minutes later. He had tried formula before to supplement and he had never had that happen to him previously. It came all of the sudden and it was not a big reaction to it, jut the reddness.
The doctor suggested to use soy formula and that worked. She said that sometimes they grow out of it by the time they are toddlers and my son did. She is now 4 and drinking regular 2% milk. However, since as a baby they don't really get tested and we did not know exactly at what component of the formula he was allergic to, I am allways cheking for signs on any lactose sensitivity.
Good luck and I hope you find out what it is soon. It that sound like he is having an allergic reaction to something.