Our son got hives a few years ago. It started on the back of his neck with what looked like mosquito bites. Within two days he was covered pretty much head to two with them. Thank goodness they weren't itchy because he would have been a miserable little man! It took about a week before they were all gone.
Yes, it probably an allergic reaction to something. We saw a peds dermatologist because our doc had never seen such a bad case. We were told it was probably an allergic reaction to something he had eaten. She said hives don't typically form from a contact allergy. But, I think it was a new lotion that I was using. He started getting the hives a day or two after I started using the lotion. I only used it for about a week before I decided I really didn't like it and stopped using it. That's about when his hives all went away.
Probably the most the doc will do it prescribe something to help,with the itch. But watch him very closely! If he starts to have swelling on his face or throat or trouble breathing get him to the ER as soon as possible!! Also, be very careful not to give him more then the recommended daily doses of any over the counter meds.