It may be unrelated - I disagree with Life360 only because herpes is typically on the outside of the mouth - but - if it's small and sore - it could be a canker sore which is your body's own internal immune response and not caused by a bacterium or virus. If you're kissing extremely rough - it could be irritation as well.
does he use a bees wax chapstick
Have it checked out by your doctor. Im not a doctor but could be a herpes virus.