Is it sore or tingly? If it is tingly it is likely a cold sore (herpes). If it is herpes your boyfriend probably already has it. Most people do even if it is dormant and they don't know they have it. It is also passed on before the sore appears, the most infectious time being just before the sore appears when it is tingly. But it can spread even before then too, when there is no tingle and no sore, so you really have no way of knowing. I had it since I was a kid, either from sharing cans of drink with people or from a relative kissing me. It sometimes looks like a bit like a crack when the sore first appears, but usually it will turn round shaped and become scabby. I think if yours doesn't go like that, it is probably something else. Blistex is a good lip balm.
try using a good lip balm, not wax or petroleum jelly (vaseline). It could be a cold sore (herpes 1) or just really bad chapped area. HSV 1 is very common. It can show up unannounced and direct sun light it known to cause it to spring up. cold sores will clear on their own in a week or so. try not to lick or chew or pick at that spot.