[Solved] What rights do stepfathers have?

Brief summary of our situation...

My partners ex-husband left her, and their new born baby boy, for another woman and, quite rightly, she divorced him.
A year later we began our own relationship.

Initially, my partners ex-husband saw his son fortnightly (his decision) but rarely had him overnight and on a few occasion brought him back early (it later transpired that this was because some of his times with his son clashed with his passion for drum and bass and clubbing). Even at that though we accommodated him - everyone deserves a night out sure, but maybe his timing could have been better and as for his priorities...

Eventually, he didn't see his son at all, nor did he make any contact. When contact was eventually made, he would be verbally aggressive on the telephone, death threats to myself, insults to his ex-wife. This continued so we involved the police and he was charged with a telecommunication offense, as I understand it. If anything this made matters worse in terms of his attitude and behaviour.

He also, around that time, ceased any payments he was making towards his son which were, previously, informal and were half of what the CSA estimated he should be paying. So...my partner involved the CSA and they successfully chased him up for overdue money and set up an on-going payment plan.

He has now left his employment and the CSA, again, are having to chase him up because he is making no effort to contribute but is claiming that he is self-employed and doesn't earn enough (the clubbing continues though - Facebook can be helpful sometimes!).

He hasn't seen his son for over 14 months now (the boy is 3 years old) and his behaviour is still, as far as we are concerned, unacceptable. He has threatened me in the local shop, been aggressive towards his ex-wife's mother and his ex-wife.

We are both now increasingly concerned about the future and I wanted to ask advice on what rights we have and also what rights I have as a stepfather who has been involved for almost 2 years now.

We wrote to him last year, and copied that in to a solicitor, asking about his payments mainly but also expressing concern about his lack of involvement - no reply was forthcoming.

Personally, I don't want him to have anything to do with his son whilst he is in this state but I feel helpless because I'm not aware of my rights. In fact, I'm very concerned about any involvement his father has at all because his behaviour has become almost delusional and I have no doubt that this would be very destructive for the wee fella who is a terrific young boy brought up by two people who love him, and support him, very much.

Where do we stand?

What, in your opinions, would be the best way forward?

I am paying half of the boy's child care costs (we both work) as well as, obviously, providing a home, heat, food, days out, etc etc. I feel very involved in the young boy's life and that is starting to become very fatherly but, at the same time, I don't want to come across as the child-snatching stepfather because that is playing right into the ex-husbands "they're trying to take my boy away" script.

It's a very difficult situation and I feel a bit lost with it all.
I'd really appreciate some advice.
Thank you.