There’s a guy I’ve know since school who I’ve practically spoken to online every day for the last 10 years. We weren’t particularly close at school (left well over 15 years ago now) but we got chatting online not long after and I can honestly say he’s now my closest friend, and he says that I mean as much to him too. Though we’ve been close during those 10 years we hadn’t actually met up again in person until about a year ago and that’s when our relationship turned sexual. The thing is, whilst I’m single, he’s married and has been for the last two years. I’ve never met his wife, or spoken to her, but I presume she surely knows of my existence (as his friend, at least) as I’m constantly chatting to him via messenger throughout the day, surely something she’d notice?
When we first met up last year he explained that he and his wife had an open relationship and that she’d be ok with us having sex as she was also seeing other guys, which I believed as he’s always expressed that they’re quite open and in to all sorts of experimental ‘kinky’ stuff. Our plan was to have a ‘friends with benefits’ sort of relationship but as we’ve grown closer I’m starting to feel confused about my feelings for him, especially when he says things like he’d start a relationship with me straight away if his wife was to ever leave him for another man. 
I recently started to question his honesty and I wonder if his wife truly is open to the idea of him seeing other women, as he recently explained that as long as she doesn’t know the details then she’s ok with it (and same with him not knowing about who she sees) which seems odd but I guess plausible? He’s expressed that he and his wife are going through a rough patch of late and not actually having sex together, but also that he has suspicions she’s currently sleeping with her boss, which he says doesn’t bother him but I get the feeling it does. The thing is he confided with me that she had to take a pregnancy test recently (which turned out to be negative), and it just rang alarm bells with me cos either he’s lying about them not having sex with each other (why, I don’t know) or he’s lying about her sleeping with other men cos surely he’d be questioning the paternity if she was (which he wasn’t). 
If this was some random guy I would probably think he was playing me and his wife for fools, but he’s my best friend that means the world to me and I just don’t know what to think. Am I right to be suspicious? I don’t want to call him out and risk losing the friendship we have but if he hasn’t been truthful about his relationship being open then I’m left with no choice but to end it.