Long time lurker but just recently started posting 
I need some outside perspective as mine is, well, clouded with emotions (and not nice ones).
I’ll try to keep it as brief as I can as to not drip feed but I may fail miserably so bear with me!
My exH and I have been separated for 6 years, divorced 2. We had a DS together who lives permanently with me as I have sole custody (more to that later).
Our DS never saw us being together as we separated when he was basically a toddler. My exH is not a bad person per se (although the bar is very low on this) but he has put me through so much shit it’s unreal. He ruined my life and I am still in therapy to make sense of how have I allowed him to happen to me. Married in haste, repent at leisure is basically the story of my life with him.
Yes it’s all my fault for ignoring all his red flags, worse than a soviet meeting.
Yes, I made excuses to his appalling behaviour every single time (“he is tired!” “Noooo he is not rude it’s just that he is shy” “you got to know him to fully appreciate him” “he is a bit closed up” “I think he may have undiagnosed autism” etc). All my friends and family told me, gently and not so gently, that he was bad news but I was too dumb and in love to engage my brain…
Anyhow.
The icing on my shit marriage cake happened when one fine day the police in bulletproof stormed in my house with a search and seize warrant - my ex has been accused of possessing indecent images of minors. He didn’t get arrested only because the policeman checking his laptop realised that yes he may have downloaded stuff but, to be fair, he had his laptop stuffed with tv series, movies, tv programmes etc - for the record, we had full Netflix and virgin Media subscription at the time.
Long story short, I booted him out as soon as the police left, changed the locks, he cried that he was about to kill himself, tried not to yell “if you don’t kill yourself I will!” But called the police to report this potential suicide threat.
Eventually they helped him, gp referred him to a psychotherapist or something like that.
Now, my exH is the type of person who loves nothing more than a pity party. He also needs to be more sick than you. If you tell him you have a headache, let’s say, he’ll immediately tell you he has been having a migraine since 3 weeks ago, his neck is in pain, he may lose a limb and may have lung cancer.
He didn’t like that I was (really) suicidal for 3 years due to post natal depression (he didn’t believe it is something it exists but rather that I was moaning and using this poor excuse to be unsympathetic with aaaaall his illnesses and struggles etc).
Anyway, so off he goes to the therapist and he tells that he is severely depressed because I broke his heart ( - for info - he was cheating on me with more than one colleague before I pull the plug, but told me it was just work banter… yeah right) and because I was not having sex with him he became addicted to porn 
and to downloading stuff from the web 
.
If anything else came out of the meeting I don’t know, all I know is that the therapist rang SS and SS rang me straight away “to help me cope with this situation”.
Unfortunately I must have a different idea of help as they grilled me and my son for 4 months, trying to catch the both of us saying something / doing something that they could regard as “we need to remove DS”. They failed but they made me sign a contract with them where they imposed I would only allow DS to see his dad under my supervision (or of someone else I trust) if and when I wish for this meetings to be held pending review after a sentence is issued.
Now, this was 5 years ago, no sentence, no trial, no news from anyone. We are not in the U.K. anymore e neither is my ex but we live very very far away (plane journey, to give an idea). His choice not mine.
Our DS is a brilliant boy but he has his own struggles: he likes his routine, he takes everything face value (if you joke he gets offended as he doesn’t get it, for example) and he had lots of insecurities and needs reassurance. One thing he struggles with is if you promise something and don’t keep the promise - something neither me nor my partner (who my son regards as his dad more than his birth-dad) ever do.
My ex-h has been told million of times to not promise shit to DS if he can’t then keep it. Yet he does it all the time. He knows full well that he can’t get our Ds to spend holidays with him or whatever, and yet put in our child’s head that he’ll take him here, there and everywhere.
Latest, DS one day begged me and begged me cause he told his dad 
that I’m going to take him travelling to a city he absolutely adores and his dad said “ask your mum the dates and I’ll come too or better I’ll take you myself”.
Ok, my ex H and I do not communicate unless it’s for super important stuff regarding our DS (such as sign this form / document etc) but why the bloody twat can’t just zip his mouth and text me first “hey DS told me this what do you think if….?”.
I had to be the bad guy telling DS no, because covid and then we will decide last minute and other pathetic excuses (and then I reminded him to not tell his dad about our stuff).
I’m tired of having to mother my ex and tell him “stop doing this” cause he can’t engage his brain anyway (though I doubt he has one), and I’m tired of keeping my son in the dark about what a moron of a father he has because, despite him being a turd, DS has a right to have a relationship with him… so I guess my question is:
How can I help my son understand that his dad made some very poor life choices and hence why he is so far from his life and please take what he says with a pinch of salt as it may never happen?
I love my son to bits but sometimes I wish I never met his dad….!