Abandonment Proof

How do I go about proving abandonment? I am in California and just starting the process. My husband is adopting my ten year old. The BF has not seen him in over ten years do I just show family pictures of my husband with my son growing up and it is up to the BF to prove he has had contact (which he hasn't)? we had a rough patch about six months ago and the state went after him for support and I think his current wife then found out he had another kid he hadn't told her about and will not sign just to despite me for his wife leaving.