Maybe she is and maybe she's not, but lying about a pregnancy is a strategy to keep a man that's as old as time. I mean think about it. In her mind what would be the most 'likely' reason for him to block her? He wants you back. The reality of it is, if she is pregnant, that he's bitten off much more than he can chew and he's taking the coward's way out by simply refusing to deal with it. My goodness, he's certainly blown his whole happy life to shit, hasn't he? GOOD!
By contacting you she's using both barrels; making you even more furious and determined than you are now to never take him back AND hoping to 'force' him into contacting her if only to tell her to leave you alone.
I think at this point, all you can do is add this to your 'sit and wait' list. Either she is or she isn't and only time will tell. You can deal with this when you know for sure. Don't borrow trouble. Her life is not your problem. She made her bed etc etc.
As far as what you do with this information, that's up to you. I wouldn't tell his mother. What would be the point and why should you tell her news that he should man up and tell her. Let him put on his big boy pants and deal with her. And as far as telling him, you know, honestly my very first reaction was "Nah, he's only going to lie or it's going to end up in a huge 'discussion' with him making excuses, more lies, and a temper tantrum or crying jag". None of which you need to deal with right now. So, I'd put it in the same box with the rest of the shit he's dealt you so far and let him deal with it on his own. Concentrate on the 'practicalities' right now. Let tomorrow unfold itself to you as it will. You'll deal with it then.
And YYYY to getting a Ring doorbell! You can get battery operated ones that just stick up with super strong 2-sided tape. Not that expensive and sooo worth it!!!! We love ours and knowing that we know who's at the door and can decide whether to answer it or not.