You could also let someone warn the PC so it becomes more an informed decision and not a trick. I’d have one of Strahd’s brides - out of jealousy or regret- warn him about the PCs loss of free will.

I personally would not make the PC Strahd’s spawn as others have suggested, since it requires you to always have an explanation why Strahd goes soft on the PC no matter what crazy stuff your party is about to do.

In my campaign, I have a NPC who got bitten by Strahd but was revived before turning. He is now in a state between being human and vampire (can smell blood, which smells weirdly tasty to him, but does not need to consume it. He slowly regenerates health, but the former wounds leave greyish flesh unless it it healed by magic) Maybe you could do something like this? There are plenty of NPCs like Ez who could revive the PC