Need advice on non involved father

I would like some advice: to cut a long story short...I fell pregnant to a guy I was engaged to when I was younger. The relationship ended when he gave me the option of choosing the baby or him and an abortion.
I was almost bullied into the abortion and the bullying carried on throughout my pregnancy.
Because I was young I took the advice of not putting his name on the birth certificate (he was threatening to take the baby from me).
A few years down the track I learn he had a much older lady I worked with pregnant at the time and they had a son only a few months older than mine.
The problem is: years later they enrol their son in the same single stream school my son goes to.
I have expressed my disappointment in their decision considering the boys are the best of friends and to look at they look like brothers!
My sons father refuses to have anything to do with him, recently my son was going through some behavioral issues and I briefed the father on them and left him with my number twice! And twice he didn't make contact.
It breaks my heart! My son knows who his dad is, the boys have even discussed being brothers at school...I don't know, I guess seeing the fact his father can walk straight past and not even look at him makes me sick to the stomach.
What would you do? I've tried talking to him reasonably, I've put myself out there, I've said 'talk about it and let me know' and nothing has come of it. What would you do? What do you think I should do? Any advice would be appreciated!
Oh-I now have 2 more children and a wonderful partner. I'm not angry at my oldest sons dad, I can forget the bullying and abuse I received while pregnant, I don't think of him as an ex, I just want him to be more involved with my son.
Sincerely
In a rut!