I think two things happened:
1) as time is passing, the imprint is slowly wearing off, causing its effects to weaken.
2) Hen is having sex with Ivan (and Alex), which causes a more "lingering" medicine to the effects of the imprint, as opposed to simply being near each other - reason for why he can stay away from Ivan for prolonged periods of time now, when before he couldn't.
I agree with all that except he hasn't had sex with Alex(he's the albino snake right?) yet.
I nearly put a * there but yeah, I kind consider them doing... whatever it is they are doing lol... as helping Hen with the imprint's effects, that's why I put it there as well ٩(๑❛ᴗ❛๑)۶
So I have a question. In the beginning of the story after the hen was "imprinted" he could not stay away from the snake for more than a day or so without having negative side effects. But it seems that they weren't worried about the negative side effects in the past few chapters. Did the imprint wear off or did hen start taking the pills?