Astarion's missed opportunity

The fight with Cazador kicked my ass the first couple times I tried. I ended up having to restart because Astarion died. Sad face, whatever.

Decided not to go nuclear, Karlach was so proud. But I realized there were a few game mechanics I wanted to try out and see if I could change some things. (The Holy Water I had been walking around with for 2 whole Acts did almost nothing, btw. Shame.) I went back and tried to get Astarion to bite Cazador and I can't? I 100% should be able to make him a "full" vampire by biting Cazador in combat. Aren't we told in Act 1 that that's all it takes to graduate from being just a spawn? I've had my heart set on that ending for him for well over 130 hours of game play and I'm actually pretty disappointed.