It sounds like an allergy to me ... my son, just like many other Moms', has allergies to various things. I would definitely get him in to an allergist to see what the severity is and what to do to make him more comfortable. Hives are rather unpleasant!
I say an allergist, because my son had problems, for a WHILE, that our pediatrician kept blowing off as other things (he didn't have allergies, my 6 month old still had newborn rash/acne ... it wasn't allergies it was colic ... and the list went on and on) It took having 2 instances of, my then 15 month olds, eyes swelling shut and an on-call Doc to finally get a referral to an allergist. We now have a great allergist, a multiple layer allergy defense system and a new pediatrician. All is right with the world.
So, S. follow your gut, you know your little guy better than anybody else.
All the best!