You've known him a couple of months, your friend hasn't heard anything about him sleeping with the ex for a couple of months - so maybe he was and stopped when he met you. As for being different with you in a crowd, I went out with a colleague before and neither of us wanted anyone at work to know for a good while. This meant we were different around other folk, and I did take it personally sometimes at first, but then I realised we had to be that way if we wanted to keep it to ourselves. Once we knew where things were going with us we told people and could be more ourselves. We've been together for 6 years and married for 4 of them now.
Just talk to him and trust your own judgement rather than what friends are telling you. If he says he's not with his ex and you believe him then go with that. As for the distant thing, it'll resolve itself in time.