This. Oh sweet lord of all that is holy this.Batou667 said:Three words: dump his ass.
If he does still have feelings for her and has kissed her while he's been with you, then not only has he fooled around behind your back but he's also lied to your face about it, several times. Now I like to think of myself as a forgiving person, so I'd be prepared to let the fooling around part slide once or twice. So long as they were completely honest and genuinely sorry. But even I would not tolerate him any more.
If he doesn't have feelings for her and from his perspective he's just being friendly, then he's still an arsehole. He's not making you happy and when you try to tell him this he just bats away your concerns and makes you think you're being paranoid. That shows he cares more about keeping his ex happy than is current girlfriend. Even if noting has happened between them, it is still not ok for him to be treating you like this.
Get the hell out, before he has a chance to hurt you any more.