Well first of all, he should stop using the product he's allergic to.
I'm allergic to product with fragrance in it. I break out on my face, neck and shoulders. So I buy unscented or low-scented product. It can sometimes be a pain in the butt to find, but it's out there. Just lurk around the hair products aisle. If you can't find something completely unscented, look for something with a lower scent profile (Aveeno has some). Also...does he really need a "wax" product? Could he get the same results with a mousse (which would probably not leave so much residue on his head all day)?