I'm re-reading this and trying to get if there was actual cheating going on, or if you just think he might cheat. Only your title says "being cheated on," but nothing in your text says "I found him in bed with another woman" or anything like that.
If he hasn't had sex with someone else, you haven't been cheated on. If he hasn't cheated, the only alarming part of what you originally wrote is that he is "going on dating sites," because a dating site is for meeting people and possibly actually going to see someone else. But all you say he did was make flirty remarks. I think it would be exaggerating to call that "cheating." If that's really just it, and he hasn't been stepping out on you and having sex, I'd drop it and for heaven's sake, stop checking his social media.
So listen. I don't tell women that they have to leave. Many do make things work after infidelity if they think they want to salvage the relationship. It takes work. Has he actually been intimate with someone else or are you not really sure but think he has? But regardless, he's emotionally intimate with other people outside of your relationship. Whew, hard to swallow. Does he know that you know? What does he say? His reaction is telling as to if this can work out or not. Contrite and willing to work on it gives you a chance! Then he needs to do that work. And you need to be able to voice your fear, concerns, anger and insecurity about all of this! A couples counselor or a counselor all your own to talk it through with would be helpful! It takes time. It doesn't heal over night. And hopefully he is committed to the hard road of regaining your trust. I feel for you. This is hard. How did you meet him if I may ask?
You're putting up with an awful lot from him. If he's truly addicted to porn, how is he "dealing" with that? Is he getting help? It doesn't sound like it. And flirting with someone he has a past with isn't part of a porn addiction - that's slimy.
If he's addicted - to porn, to sex, to whatever - what is he doing to get help? What is he doing to prove to you that he respects you and wants you to stay?
Whether or not he's cheating - and everyone has different definitions of that, and if you haven't defined for him what you think it is, now is a great time to do so - he is still doing things that make you feel bad.
When you get to the point where you are checking his socials, it's bad. You have a child together. I'd suggest couples therapy. If he won't go, go alone.
Best of luck to you. :)
First of all, none of that is cheating on you. Yet. Maybe he is and maybe he isn't, but you'll never be able to police a lover's thoughts. I'm also not sure what a porn addiction is -- what's the withdrawal when you stop? It's a bad habit or a neutral habit, I'm not going to judge it people liking porn and I'm not even sure what porn actually is, but most of us at some point do look at something that is sexually arousing to us that isn't our wives or lovers. Doesn't mean we're acting on it, which is what cheating is. I would say, though, that if you can't trust him, the relationship is already over. Can't be love without some blind faith to it, because humans are pretty poor specimens of wonderful behavior. If you truly can't trust him, again, it's already over. If you're not sure if you trust him, that's an issue you have to work out with him. As for being perfect for him, there's no such thing. We can and usually do love a lot of people before we're dead. Few have just one love in their lives. If you want to work it out, that's what you will have to do, work. That will involve the two of you talking it out, or couple's therapy if that isn't possible, but I do want to say a third time, if you completely lack trust in him forever and ever you've already broken up with him in your mind so you have to decide if that's really true. So far, you've said he's done things you wish he wouldn't, but haven't stated yet a case of cheating. Peace.
Do you have access to a counselor or therapist? Cheating is a really hard thing to get over just by yourself, you need a neutral third party to talk to.