Sounds like he has all the symptoms. Also sounds like she may have dumped him, so now she's not an option he's preparing for her outing him, not just by making out she's nuts but also by making you like him again. Was there a time (when she dumped him) that he was especially weird/sad/coming home late?
My ex did the mentionitis, too, but telling me about this amazing work colleague who had done such amazing things and he was so impressed. I think he just had her on his mind and it came out.
As the horrible behaviour got worse (dead eyes etc.), I would call him out on his behaviour and he would say "You don't know anything!". I thought he meant that I didn't know what was going on in his head or understand his emotions.
After a year, I got into his email account and knew everything. But I didn't tell him straight away. I started saying things he'd done, but as if I was guessing. He reacted with the usual "You don't know anything!" and I realised that what he had meant all along was "You can't prove anything".
They think they are safe if you can't prove anything. That they can say this stuff right up close to the border of you finding out, but you can't prove it, so they're safe.
When someone says you're paranoid, the instinct is to deny it and be super unparanoid, because you don't want to be a horrible person. So the more he calls you paranoid, the more you try to be reasonable and give him the benefit of the doubt.
It was only when I got proof, and it was exactly what I'd guessed and much worse, that I realised I didn't need proof. And that I'd been so busy "not being paranoid" and explaining away each individual symptom that I'd never really considered how much more obvious it was when you looked at them all together.