My dc are still much younger, and I may be over reaching my expertise but ime, it pays to take the time to listen and really hear what’s going on for them, rather than launching in with judgements.
Try to get him talking about what happened with the mother of the child and what’s going on with his current ex.
I don’t know if you’ve ever come across the idea of restorative practice (it’s worth a google) but questions like these are a good framework to follow.
What happened?
What were you thinking of at the time?
What have you thought about since?
Who has been affected by what you have done?
In what way have they been affected?
What do you think you need to do to make things right?
I’d be taking the approach that he’s my son, that I’m on his side and want what’s best for him (because ultimately stepping up as a responsible man of good character will do him more good that weasling out of his responsibilities). These are the times when our dc need us most. He’s acting like a scared and stupid boy and it’s time now to be a man, but he needs guidance, wisdom and love to find that bridge and cross it.
There’s a huge social element at play here. I’m not saying you can shrug it off as society’s fault, but it feels very lonely and overwhelming at times to be trying to instil good values in our dc when the wider culture seems to promote the opposite. There was a time when those messages were echoed in school, in church and from govt but we’re living in a decadent, licentious society.
Good luck op, I know it’s a shock, but you can do this.