Need some advice. Bio Dad had contact for first time and left Son in cinema for 45 minutes alone.

I need some advice. Bio Dad has just had contact back with his son and was having an unsupervised visit. He took him to the cinema to watch a new film.Son came back and said "Daddy left me for 45 minutes so I could get into the film because I was being silly". Dad went outside for 45 minutes and left him. Son had to leave the film and go outside to find him.He then tells us "Daddy used to leave me in the house when he went to the shop when I stayed at him". Son was 7 and 8 back then. Son his 9 now.He's supposed to be building our trust to start having son overnight. But this is winding me up! He can't take care of a child.He lost contact after he went to prison for violence. Mom said this is the last chance and he has to build her trust before she agrees to overnight. On his first unsupervised contact with son he does this.I am fuming. I don't know what to do. I'm just stepdad (8 years stepdad).Is what Dad did acceptable?Edit: I want to add some context to the prison situation. He was sent to prison for beating someone up with a brick. The lad he beat up nearly died. He was on bail for this, but he then smashed up his girlfriends house and was sent to prison for breaking bail conditions.I spoke to bio Dad yesterday alone. He was accusing me of trying to stop his contact so I met him at the bus stop alone and I had an 1-2-1 chat about this and told him to stop making accusations. He knows the reason why his contact was stopped was Mom's decision but he insists on blaming me. So we got it sorted and then we were randomly chatting.I asked "Keeping out of trouble now?". He joked "No. Sometimes people deserve a slap". I said "Surely Son is more important though? Why not try and find some work and get life back on track?". He responded "I'm sick at the moment going through treatment. Some days I feel like just beating someone up. Do you ever feel like that?". "Nope, I don't" I replied. I said to him "Just remember its Son who suffers if you go back to prison and Mom won't allow contact". He then said "I know. That's why next time I will make sure I do it off camera".So he's gone to prison, nearly lost his son in the process and hasn't learnt a thing. I told Mom "Give him the noose and let him hang himself, because he will". It's just so sad we have to go through this process and I know Son is going to get hurt.We're going to speak to a lawyer and make this formal. via /r/Parenting http://ift.tt/2iIUcc7