Hope you’re ok OP. If you’re 100% sure it’s him I think you need to let him know how it feels to be messed around like that. I would arrange to meet him then just never show up. Waste his time, as he has wasted yours. I wouldn’t send him a screenshot or anything else, the not knowing (or not knowing how much you know) will be worse.
A few years ago I was at my (then) boyfriend’s flat and he was in the shower when his phone pinged with a message from one of his (many) female friends. My blood ran cold when I saw it - she was talking, in detail, about how great the sex was between them last night! I opened up his phone and found tinder, loads messages arranging dates, you get the idea. I took pics of everything on my phone then when he came back into the room I didn’t say a word.
I had previously raised suspicions of cheating and he strongly denied it, so I decided there was no point in having that conversation again. I also knew he had cheated on other women before but stupidly thought he had changed his ways! (I know, I know). I decided that this time he needed to learn his lesson. So I didn’t tell him I had seen anything, kissed him goodbye and left for work.
That week I orchestrated my revenge. We were due to go on holiday at the end of the week, so I set about:
- encouraging him to book expensive and non-refundable activities
- cancelling our hotel (in my name) and everything else I had booked
- swinging by his flat to pick up my belongings (told him I needed to pack them)
while all of this was going on I made sure to carry on as though everything was fine. I told him I was busy at work so couldn’t see him, but still had long phone chats etc. Hearing him continue to lie to me during these chats (telling me how much he loved me etc) was quite helpful in helping me get over him.
The day we were due to go on holiday I told him I would see him at the airport. He texted me saying he had arrived, asking where I was. I told him I was on my way, running late for a while, then just stopped replying and blocked him! I often think of him sat at the airport with his bags, trying to work out WTF happened. No idea if he took the flight and discovered I had cancelled everything I paid for, inc the hotel!