Is it common for a BPD partner to suddenly switch off sex? We were fine two weeks ago. Now it suddenly won't work. He claims there is no one else. Is this a common type of manipulative behaviour? I believe he is trying to manipulate me.
Yes, although the BPD behavior tends to be more subtle and convoluted than simple manipulation. There will be 3-4 layers to it when you finally figure out what's going on.
The kicker is that if he's like my pwBPD once this is all over he will tell you that
you stopped wanting sex for three weeks, and how you were to bad and hurtful to have done such a terrible thing and he has been lying awake for the past three weeks wondering how you could be so cold and unfeeling as to have not had sex with him that whole time.