Morning 
It really is OK to mourn Poppy. The end of any relationship of any depth, however good/bad, is another milestone in your life. Have a good cry, have a hot, sweet cup of tea. Eat whatever you fancy - just try to keep your blood sugar levels up. Post on here. Talk to friends / family. Walk as much as you can. And then have another cry again and repeat the process, as many time as you need too. It'll get easier, believe you me, it just takes time and that's something we none of us can hurry up.
My intuition is that your STBEx-DP is lying about what was recorded on Alexa, and you know what? Even if those things were said, even if some of your son's friends were getting intimate while you and your STBEx-DP were away, they're not the first teenagers to do so, and they definitely won't be the last.
Your STBEx-DP is just trying AGAIN to make you feel bad about your son, to doubt your son, to feel bad about yourself and to doubt yourself.
Ignore him It is such a petty, trivial thing. And if your STBEx-DP was half the man you hoped he was, he would quietly take your son to one side, have a constructive and supportive man-to-man chat with him about such matters, and deal with it that way. But he is either unwilling, or incapable of doing that. So he's no good to you or your son as a quasi-step dad, or even Mum's lovely boyfriend. Because he just isn't lovely. He's horrible.
And, you know what? Thank you so much Poppy for all the kind things you've said about me posting on here which are helping you. And also thank you to LizTaylorsFabulousTurban and Arkengarthdale
. And Hermonie2016 so much of what you have written has struck such a chord with me
.
Sharing what I've gone through is also massively helping me because I am externalising and recording some of what has gone on with my Ex-DP. And that's so good, because in my more teary moments, which still happen reasonably regularly, it would be so easy to only remember the good bits, and to slip back into self-doubt mode - was it my fault? What if I'd done this? Said this? Etc etc etc.
If I can give you any advice right now is block out his emotional rubbish and let him get on with moving out today. Focus on any practicalities - and resist the urge to help him. Get on with your own to do list today. And make plans to have a friend over this evening - start doing the things that his selfishness has prevented you from doing right away. Believe you me - IT FEELS SO GOOD! 
You can do this We're all right behind you.