Swollen lips after kissing

Hi looking for possible suggestions on what the problem might be here.

So I have been dating a man for around 2 and a half months and after every time we see each other about 2 hours later his inner lip/mouth gets really red and swollen. I’ve tried a few things to see if it will stop such as never biting lips, not wearing fragrance, brushing my teeth right before we kiss and nothing changes. I don’t get a swollen mouth or anything afterwards just him. Tonight I ordered an sti test for chlamydia and gonorrhoea to rule out those things possibly. Just wondered if anyone had any other ideas to find the problem? Also he doesn’t have any allergies we know of and the swollen lips only happens when he sees me.