Yabu to have split in pregnancy and still to have put him on the birth certificate.
Yabu to let him him have unsupervised access to an 11 week old who should be with his mother.
Yabu to expect a cruel violent man to be kind to an 8 year old. Of course he was mean, he will do anything to hurt you.
Yabu as you have started this situation of visits only for your younger son, a pattern that you will probably allow for the next 18 years which will affect both your sons and how they interact with each other.
Yabu because you had a chance to break free especially as you had contacted social services but yet you gave more power and control to an abusive violent man.
I'm sorry that sounds harsh but once a man is abusive, you do yourself and your children a favour and stay away. I know it's hard as I've been there.
You broke up, that was you chance but you didn't take it, you made a stupid decision to put him on the birth certificate, why on earth would you?
I'm sorry op but I think you need to wise up to how you decisions may pan out for the next 18 years.
Stop expecting anything from this man and stop allowing him into your and your boys lives.