Thanks for your reply SJL. I ended up continuing with the zyrtec everytime I saw more hives. It took just over 48 hours and then settled (just BEFORE we were going to go to the doctors - of course!) so I didn't end up taking him. It's interesting what the doc said about the cold impacting the recovery time though. I didn't realise that it could take so long for the reaction to settle down.

Thanks again