At this age, he needs to be completely covered when you are outside with him for an extended period of time! He may sweat, but if you keep him hydrated it is better than a sunburn! COVER THE STROLLER with a lightweight blanket or sheet.
Yes, I'm scolding a little bit here, but there is no reason for an infant to have a sunburn. Yes, Tylenol for the discomfort and plenty of moisturizer to keep him from peeling, but as for prevention- be aware of it from now on and don't have him exposed! You can't use sunscreen on his baby skin at this age.