An OTC antihistamine should do wonders for him.
Just a word of caution about Zyrtec. It made my 5.5 year old daughter violent. She was on it a few months. I just thought it was her attitude and that kindergarten was coming up, but it got to the point where we took her to an OT for an eval thinking she had ODD. On a whim, I took her off of the Zyrtec, and once she had been off for about one week, she was her sweet self again. I was so pissed that I hadn't thought of the connection sooner! Happy she was better, but so upset that I had essentially lost that time with her. We use Claritin now. I had her (then) 15mo old brother on Zyrtec and his allergy symptoms cleared up, but we took him off of Zyrtec when we took big sister off of it, and he does just as well with Claritin. I know this doesn't happen to every kid, but now that I know what Zyrtec can do, I never recommend it to anyone.
Wife to my hubby since 2004, SAHomeschoolMommy to DD1 9/2006, DS1 11/2008, DS2 1/2011, DD2 7/2013