protecting my son from ex wife new partner

What is the order i can get out on my ex wife partner seeing my son due to he has been violent towards my ex wife. She refuse to go to the police but I have seen the bruising on her and I have messages saying she is scared of him and she told me what he has done

There isn't an order that you can get for your ex-wife without her agreement or cooperation. If she agrees to she can get a "non-molestation" order that would list the things but her partner isn't allowed to do but clearly he already is physically abusing her which he isn't allowed to do anyway so the first step as you say is for her to end the relationship then if he continues to harras her contact the police and get the court order. All of this though would have to be done by her and if she is unwilling to do that then would you not be better off if you want to protect your son from being in that environment to apply to have custody of your son at least on a tempory basis until she resolves the issues she's having as it can't be a healthy environment for your son to be in.

Thank you for the advice but unfortunately she is blinded by what he says so she won't help.

Are you in a position to have your son more and even telling her you are concerned and are going to make an application to have primary custody of him may focus her mind a little and spur her to make the changes she needs to? It might have the opposite effect and she starts to become difficult about contact but that's something for you to judge but either way the only way you may be able to protect your son is to remove him from the situation.

We agreed to do everything 50/50. This all came out new years day when she messaged me at 3am saying she was scared and can she come to the family home. I said yes as she is the mother of my child and I want her safe for him.
I have told her not to go back to him as I fear what he would do to her and I'm the one who would have to tell our kids (she has 2 from a previous relationship which I have raised for 14years) what has happened to her.
I have been told about a Prohibited Steps Order but the last thing I want to do is push her more towards him

You are doing the absolute right thing by being there for her. I was in an abusive relationship and like her did not want to take it further at the time. Unfortunately I hid it from my ex husband so had nowhere to go. Keep your door open, be supportive without judgement and hang in there, she will gravitate to safety. As for your son, maybe offer to have him more to keep him out of the situation until she sees the light

If you tell social services your fears they will step in to ensure your wife understands her continued involvement with this man is putting your son in danger. It doesn’t even have to be that he might be physically hurt, a dim view is taken of children witnessing abuse. Social services can put their own conditions in place if she wants to continue having your son stay with her