This sums the situation up perfectly. Yes it's sad that the little boy doesn't have the same family unit as the 2 toddlers but that's life. That's just what happens when people split up.
The girlfriend has no obligation to your son, however dad should definitely be making time for him since he only sees him once a week. Having 2 toddlers must be a lot so I don't think you can blame the GF for focusing on them. Dad should however take his DS out to do something more age appropriate while the GF deals with her kids.
I think it would be worth having a chat with the ex and letting him know how his son is feeling. But you need to take your feelings about your ex's new relationship out of it.
My DH was with his ex for 12 years and never wanted to marry her. He told me he wanted to marry me after 2 months, proposed after 16 months and we were married 8 months later.
It's not that you did anything wrong it's just that you weren't the one. Maybe the relationship was good and there was no obvious reason to leave so he stayed, got married and had a kid because that's what is expected in a LTR. But ultimately there was something missing. No one's fault although that can be hard to accept.
I was married previously and at the time thought he was the one. We divorced because he cheated. When I met my DH he absolutely blew me away. Every previous relationship felt like nothing. This was proper fairytale love and I'd never felt anything like it or been so sure about someone. I knew I was going to marry him after 2 dates.
You need to stop cutting yourself up about why your marriage didn't work. Stop comparing yourself to the new GF and ask your friends to stop passing on stories from social media.
One day you're going to meet someone who you'll have amazing chemistry with and you'll he asking yourself why you ever gave this ex a second thought.