Im insecure of my first gay relationship

I have been on and off with the guy who got me "out". He ended things three times, before we became "official". But now that we are together i am insecure about a lot of thing.
Firstly his best friend is also his ex. He claims that there is nothing romantic between them, still they go on holidays together, and i also found a picture of him with his "balls" hanging out of his underwear on his phone.
He claims to have few other homosexual friends but, (because I'm too nosey for my own good) i looked though his contacts, where almost every guy was gay. (not that he shouldn't have gay friends, but just doesn't add up with what he tells me). I realise that me snooping like this is unhealthy for me and the relationship, and I'm surprising myself, but nosy as I apparently am, I've also been looking on Instagram on what he is "liking" and most of it is other gay men. What worries me about this is that he says that the gay friends he has got is people he has either dated, or had sex with.
He also after just 3 months of being together with me, went on a trip to China for two weeks (We live in Germany), telling me he was meeting a friend there. I didn't ask too many questions, only to later realise that the friend he was meeting was another gay man who sleeps around, ALOT! I trust he didn't do anything bad, but the way I realised who he was traveling with was him sending me a picture from a gay club.

This is my first relationship and I don't know how to respond to these situations, but just now i don't really feel comfortable in the relationship. Still he seems to be fairly invested in the relationship, even wanting me to move in. Would be great with some help in how i should deal with this situation. Should i be concerned of his bestfriend/ex? is it OK to stay friends with past sexual partners? If I'm to take advise from "straight pop culture", I should have ended things long time ago.