Not my current DP, but an ex.
A lot of shady, weird shit went down the last month we were together. A weird phone call from a girl he'd never mentioned where he couldn't hang up quick enough, purposely leaving me at home when hanging out with his mates and their gfs, avoiding seeing me and having really far fetched reasons to why he couldn't. He ended up breaking up with me via text and then after some snooping on his Insta a week later, which I'd originally been blocked from, I saw a girl commenting under all his photos with stuff that was a fairly big indicator they'd been banging. He also avoided giving me my belongings back for nearly 3 weeks and I had to get his boss involved so I could get my house key, etc, off him. I think the guilt had got to him by that point.
He obviously never admitted anything so I'll never 100% know, but it would be very naïve of me to think innocently of him. He tried reconnecting with me a month and two months later, of which I ignored him. The second time was offering me gifts which was funny and pathetic at the same time.
I think at that point his fling had ended, Christmas was over (so no more presents to buy (he was in debt), and no more drug fuelled parties to go to (he broke up with me after I said I'd break up with him if he did drugs while he was with me)) and he wanted to go back to having a cushy life with a gf.
OP, trust your gut. I'd been ignoring weird shit with him for ages and it really does go to show that if you think something's fishy, it usually is. I'm not an advocate for snooping on someone's phone, but just be vigilant of what's going on. Try and suss the situation out before confronting him. If he's got something to hide, he'll just get better at hiding it. Wait until you've got enough evidence (and if you two live together and whatnot, get all your finances and paperwork in order). If he isn't cheating then that's great, if he is, at least you're prepared and you have everything you need/entitled to, to make a clean break.
Good luck!