Sorry I've written an essay! Tldr version: your DC will find the truth IF you encourage them to speak about their feelings and tell them it's OK to feel. EG if after contact they say "I don't think daddy likes me" - my mum would have said "don't be so silly, of course he does!" which just invalidated my feelings and frankly I'm 48 and still have trouble with recognising my own feelings. Better response: "Why do you think that darling?"
And now the essay!
Some people never realise the true character of a parent. I knew my dad was a cunt all along - emotionally, physically and sexually abusive. It took me until my 40s to realise that my mum is absolutely toxic as well, and in many ways her abuse has had a bigger effect on my mental health because it was just low level enough for me to think it was normal, iykwim.
My DS has been with me since I got with his dad when he was 5. His mum walked out when he was 3. She's a textbook narc. DS kept trying to make it work, he would arrange to visit her, I'd drive him there with a suitcase for a planned week - 2 days later he'd call asking us to come and get him because she was being unbearable. Screaming in his face, gaslighting him about it, being all loving in public then knocking him about at home. After his dad died when he was 13 she wouldn't let him speak about his grief, she'd just tell him "I don't know why you're missing that twat, I'm glad he's dead".
Anyway, sorry, he has now realised the true colours of both her and his dad. This was due to getting really good counselling with his school doing a referral to camhs, and then his counsellor at camhs onto a service for young adults where he saw a psychiatrist weekly.
Recognising that his dad wasn't a good parent and did a lot of stuff wrong was hard and painful, especially because his dad was gone - there will never be an opportunity for him to say "dad you did xyz when I was little and you made me feel like shit."
I have been able to give him an honest, but somewhat sanitised picture of his dad. His dad did have an incredibly hard life - but it's not an excuse, is it? I have tried to be truthful, but I have not been completely open, because there were things in my H's past that he said he never wanted DS to know about. And I've considered that it wouldn't help DS in any way to know these things, so I have kept silent.
But I have also been open with DS about his dad's many good qualities. For example, he was always quick to help anyone who was struggling, he had been badly bullied at school and that made him always want to defend anyone being picked on. And those examples are traits DS has inherited from him.
I'm guessing your ex is around the same age as you, so pretty young for a 1st child. People do a lot of growing up in their 20s, so it's possible that he might give his head a wobble and become a great dad. Possible, but sadly not likely from what you've said - he sounds abusive and actually I'd really recommend doing the Freedom Programme online so you'll be alert to red flags in any future relationships 🌸