Social security question

My sons father has asked me for my sons SS#. I told him I don't know where it is because I feel like if he wants it he can go get it himself (we do not get along). I don't remember what I had to show to obtain his SS card, it's been 4 years. So if his father goes to the SS office what info will he have to provide? I wanna say it can't just be my sons birth certificate.