My baby has had an allergic reaction. What should I do? I'm not sure what the source of his allergies is.

My baby is 6 months old. I breastfeed him and have had to supplement with Good Start since his birth. For the last 10 days or so he has started to eat cereal with formula. I have seen from time to time tiny little spots on his face at random times. I didn't think about the cereal at the time, I was thinking about the teeth. Just now, 5 hours after his cereal, I gave him his bottle and he started to get red patches on his neck and chest. The more he drank the worse it was. The patches disappeared after about 40 minutes. Should I conclude that the milk is the cause? Is there a risk that he will choke on his next bottle? I'm thinking of stopping the cereal until his appointment at the end of August to make sure it's not that, but if it's the formula I'm worried. I don't know if you can help me or if I should consult a doctor quickly.