How to deal with twat of an ex?!

I'll try not to drip feed. My eldest ds is from my abusive marriage.
I left when he was 1 but never stopped potential contact.
3.5 years on and he's visited ds once (he lives a couple of hours away)

Been with my current partner for 3 years and have another ds, he's been a part of my eldest life since he was 1 but I've always corrected him when he's called him dad, he's called 'daddy Paul' for example.

DP would love to adopt/have parental rights so I messaged exh to find out if
It's ok. He agreed to parental rights on condition my son will know he's his real dad and sees him regularly. I agreed.

Fast forward a few weeks and ignored Messages and it looks like he's just all talk - again.

I've been preparing ds about his biological dad & seeing him as he has autism and struggles in social situations or change etc.

I'm so angry and sad for him his "dad" can't be bothered.

Where do I go from here?