It wouldn't be meta gaming if he didn't try to suck your neck so early in the game cause he wants to find out if he has to still follow his masters rules and It's quite possible to do it if you recruit Karlach because you get two extra long rests but recruit Karlach early is so meta gaming it kills me.
Theoretically,
if you barely long rest at all, beeline for the goblin camp, rescue Halsin and defeat the leaders/defend the Grove in order to trigger the tiefling party, then you could start the romance before learning his status. It was easier in Early Access, but it's not completely impossible in full release.
I don't know of any way to put it off beyond not long resting and avoiding the various triggers, though.
As for bee-lining for the goblin camp also no longer works because you now have to have Astarion in the higher end of neutral to get him to sleep with you.
Which leaves me with 1 Long Rest in the Ruin, 1 in the Grove and that's it. I was hoping someone knew of another work around, cause i'd of people not having Astarion say or do anything till the Underdark. They must of been Durge's maybe.
