3 answers
You gotta ensure you have a good relationship with Astarion to see this rarity! Keep him in the party, engage in dialogues, and get positive reactions. Now, here's the trick, you gotta allow him to drink your blood when he asks, it's a mutual understanding thing. Once done, go and sleep. Yeah, that's right, you sleep! There's a chance that Astarion will visit you at night for a bite scene. Good Luck with that!
Get chummy with Astarion first, cause he won't just go for anyone's neck. Have deep chats with him, crack jokes, accompany him in journeys, he's quite a character. The day he feels comfortable, he'll pop the big question - 'May I drink your blood?'. Don't fret! Just say 'Yes' and go hit the hay. You might just wake up to find him next to you, thirsting for more than just conversation. It's a game of patience and nurturing relationships. Buckle up for some nocturnal visits!
As mentioned above, the better the rapport you manage to establish with Astarion, the more susceptible he'll be to taking your bite scene in stride. You tell him about yourself, share a few jokes together, and perhaps even invite him along on your adventures. Once he becomes totally at ease with you, he might one day ask to drink your blood. Whatever your opinion on this may be, give him the green light. Then head off to bed. And heaven help you if you will not catch Astarion at his deadliest. What a thrilling existence of life and death is camping in the wild! Do take part in rest.