Ask Google. Google is always right.

Depending on the brand, there could be numerous chemicals that could cause a reaction. Alternately, if he has a more serious food allergy and OP ate that food then used the Chapstick and maybe forgot?

To add to #5, Basically everything has chemicals in it but the chemicals that make it onto your face/body are usually through the use of body products. Now, there are so many different kinds of chemicals that you might not ever get exposed to a lot of them. In that case, you might not even be AWARE that you're allergic to something. But if by chance, you are exposed to that certain chemical. Then your immune system will think it's a threat and react. Hence the allergic reaction. In short, it could be any chemical in the chapstick. It depends on the person's body.

Could be pretty much anything, depending on what exactly he is allergic to. I'm allergic to panthenol, for instance, a skin healing agent that is present in a lot of products like skin cream, lip balm, shower gel, shampoo, etc... I can relate...my girlfriend changed her shampoo and lip balm brands when we got together, since she'd make me itch badly in the beginning and my lips would burn after a kiss.

The question was legitimate, just stupid.