why do you think he text you that? He was obviously thinking about you while sleeping with OW. Could it be he wanted you to do the "pick me " dance by making you jealous. However you now see the man for what he really is a c**t. That text alone should kill all feeling you have for him or at least make you start seeing him for what he really is. Time to ignore block and move on. You deserve better
I think you should text him again today and say "I wanted to thank you. I've been on a pre-Xmas diet, and I was about to have some biscuits last night, but when you texted me you were having sex, the thought of your naked body and inept foreplay was so nauseating that I was no longer hungry. Cheers!"
OK maybe don't send that. Just block his number and stop him getting his kicks by fucking with your mind. You can do it!