...just tell him hes full of crap,its just in his head.
and then add "...which would probably be some kind of mental disorder... if only they existed!"
I actually have a friend like this. He doesn't buy into BPD (and the ironic part is he's diagnosed with PTSD from serving overseas!!)
Then again, there are also some religious people who refuse to believe Dinosaurs ever existed.
But in truth, there are a good number of close-minded people lurking the earth. Heck, I have to call one of them "Dad" every now and then.

Part of me wants to suggest trying to get him to read a book like "Get Me Out Of Here" or "The Buddha and the Borderline" but if he's locked into his beliefs/invented reality, that might not help either way.
I'm not sure what one would say to such a person, especially while suffering from one of the supposedly non-existent disorders. Hopefully he comes around - and quick. Otherwise you're not going to get much of the support you need from him.
