Does anyone's LO have an allergy that manifests as just hives? If so, what is it that they are allergic to?

DS2 had a big outbreak of hives 2 weeks ago. We took him to the doctor and got an rx for daily zyrtec and a referral to an allergist to find the cause. He didn't have any more reactions while on the zyrtec, but we had to discontinue it to prep for the allergist appointment. Since then he's had a reaction shortly after breakfast both mornings. His big reaction two weeks ago was also right after breakfast. Yesterday I was convinced it was strawberries causing it, but he had none today and now the only thing all the meals had in common is milk. But he has milk with lunch and dinner and hasn't had reactions after those mealtimes... I know the whole point of the allergist appt is to figure this out, but it's not until next Thursday and I'd like to try and not give him whatever is causing this in the meantime, so I'm hoping for new ideas of things I could try avoiding.