Setting: Modern-day UK
I have a character who wants to impregnate someone to keep that person tied to him (said person is unaware that this character has taken this plan). He's pretty much willing to do anything (barring non-con/rape) to do it. He's incredibly intelligent and resourceful.
So far, the things I've been planning to make him do is, obviously, sabotaging the person's birth control (condoms, pills, etc.). Another is ensuring that the person has increased fertility through drugs. He has no trouble sorting out the person's ovulating cycles--that's almost obvious and elementary to him--nor does he have trouble accessing medical facilities or even that person's medical records.
My main question is: What should he do to secretly/covertly help increase the fertility/increase the chances of pregnancy of the person (who is female, btw, and is in her late 30s)? I've google-searched 'fertility drugs', 'improving fertility in women over 35/30', and related terms. I've tentatively settled on Clomid as the drug, but I'm not quite sure if this would work.
ETA: For further clarification/details: I'm quite sure if the female character has lowered fertility, but she is in her late thirties, was in a war and had incurred injuries, not to mention she got pretty banged up/hurt often from fighting crooked people, so I'm assuming there are concerns about her fertility.
ETA 2: Thank you very much for the informative answers/comments. It has certainly helped me give better understanding on the nature of fertility and reproductive coercion. Looks like I'll have to revise and reassess the way I'll be handling this plot point.