Is he cheating? Or just a pathological liar?

So a guy I've been dating since late june went to go hunting, where he doesn't have any service. He asked me to keep his snapchat streaks for him, which i gladly obliged. I sent out a snap to all of his streaks, and then i opened one of the replies, which now i'm wishing i hadn't. it was from a girl, replying "hey sexy" and a picture of her, shirtless.

he'd told me this girl was his best friend and that there was nothing romantic between them. i didn't look at any more of their messages (or any of the reply snaps from the at least 5 other girls) because that's snooping and i was afraid of what i would see.

also, one of his groups (which i saw the group name when sending the streak snaps. i really don't want to get up in his business but he's making it really obvious!!) is something along the lines of "smoke gang," and it was at the top of the group list in snapchat so obviously it had been used pretty recently. he told me he quit smoking weed in june.

it gets a little complicated here, though. we were a fully exclusive couple in june all the way through september, before we realized that our almost long distance relationship might not work well during the academic year. we decided to not have a "serious" relationship but to just try to see each other for dates once or twice a month, and then we'd fully get back together this march when i move colleges. we said we both wouldn't date anyone else.

i'm just wondering if you all see this as a problem? because we're just dating, not boyfriend and girlfriend, but we said we wouldn't date anyone else. however, he might technically not be dating other people, but sending nudes and flirting (which i personally still think is bad lol).

i don't know what to do. i can't decide if i should just suck it up and be okay with it or not, or just cut him off, or bring it up and have him think i was going through his stuff? i'm really bothered that he would lie to me about his "friend," and about smoking weed, which i don't have a problem with weed, other than that it's illegal here right now, so i don't know why he'd lie to me. i caught him in a lie a few months ago while we were still in a relationship and he swore to me that he'd only tell me the truth. obviously THAT was a lie so can i trust anything he says?

should i just drop him, no questions asked? i have no clue what to do because i really like him, but i'm hurt, and snapping naked girls doesn't seem okay.