Hi.
It's hard to know the cause of your boyfriend's sores (or whether they are canker sores at all) but they are not likely to be caused by an allergy to you.
According to Dr. Karen Rosewater, painful canker sores can appear for any number of reasons including stress, dietary deficiencies, food allergies, trauma to the skin or illness. They often occur during adolescence and are less frequent as people grow older. Some people, like your boyfriend, have reoccurring canker sores and would have them whether they kissed someone or not. It probably has nothing to do with you.
Canker sores are not contagious and are usually found inside of the mouth. They are different from cold sores (sometimes called fever blisters), which are generally found on the outer lips and are caused by a virus that can be passed from person to person.
Cold sores start out as a blister or cluster of blisters that eventually burst and crust over. They tend to reoccur because the virus is never fully passed from the body and usually pop up in the same place each time. Flare-ups take place for some of the same reasons they can occur with canker sores--stress, illness (like colds or fevers), a weakening of the immune system, etc.
It's not clear what type of sores your boyfriend has but if he gets them a lot, it may be a sign of an underlying health issue. It's probably a good idea for him to talk to his doctor about it the next time he has a check-up. If he does have canker sores, a health practitioner could also tell him the best ways to reduce the pain. Canker sores can hurt...a lot!
Take care,
