Does it say in the 5th edition handbook or monster manual that if a succubus lovingly kisses a (insert race here), he becomes a greater vampire? If not, you aren't breaking any rule-as-written.
Secondly, being someone who doesn't like to pull punches on his players, I understand, but some things need to be skirted around in order to keep the game balanced and fun for everyone. If giving the player this opportunity is going to do those, you should do your duty to ensure it doesn't happen.
Just retconning the love-kiss power bullshit is perfectly acceptable. Telling him of those symptoms and making a neat quest out of it is interesting too (as long as the party and the player think it's worth it to save him).
A similar thing happened to my characters when we were playing Curse of Strahd, and it made for quite a good quest. They were travelling back from a hag coven lair, when they were accosted by werewolves and they ALL failed their saving throws. They told lady Irena to go fuck herself and trudged off west, toward their quest goals, looking for nightshade and ergot to form a lycanthropy cure. They did this because they valued their characters and I told them through the local priest that they would certainly lose control to their insatiable blood lust and be one of them (they would lose their characters due to this).
This made a very memorable quest to them. They loved finding the cure more than fighting strahd in his stupid maze of a mansion.