Started out as joke then found out he's a cheater

So I was so happy to find someone after being a single parent for so long. Really thought the guy was a blessing. Then today as a joke I sent him a text from my tablet which has a different phone number, I sent him a funny picture and hw responded 'oh thats so true' so I text back saying 'do you know who this is?' I was about to send that it was me because I thought that he'd guess me and that would be that. Then he sends back 'Abbey?' (My name is not Abbey) at first I thought maybe he knows it's me and he's just messing with me. So I sent back saying oh sorry I think I have the wrong number. So then he texts back and asks for a photo and then sent me one of him. He then proceeded to hit on the fake me so I asked if he was single and he replied 'yes'. I then texted him from my real number and asked what he was up to and he sent back saying he couldn't talk to me because he was consoling his mother because his nan had a stroke. I ofcourse know he is lying. What kind of messed up person lies about his nan having a stroke to try and hook up with a complete stranger. I now am trying to work out how to break up with him because I can't tell him I know what he's doing without putting my own foot in it. I honestly did it as a joke and it escalated so quickly. Not only do I feel like an immature brat for pulling that crap, now I find out he's a lying asshole. How do I end it without coming out on the other side of it looking like I'm the asshole?

He text me asking if I'm mad at him because I seemed very short with him on the phone when he called me. Here's the messed up part - I'm fairly sure I'm pregnant. I do not want to be. Not now, I mean I did when I thought he was this amazing man who I'd been dating for a couple of months who said he wanted me to have children with him. I am so worried. P.s I know how stupid I am for doing that stupid joke. But I didnt expect what I got.

What do I do? We are both 25.