I am not a man. However, it is psychologically confusing for his son to see his mom and dad acting like a family. They either are or they aren't. Divorced dads often have guilt feelings over "breaking" the family they had for their children. They often see their new SO as a way of giving that family back to their kids. Sometimes, they think they need to continue to give that illusion to their children that their family is still their. They aren't teaching their children how to adapt, how to deal with changes in relationships, or reality, though. Long term, that's more damaging to their kids. Instead of giving their kids examples of healthy adult relationships and healthy boundaries, they are giving them a fairytale that won't end up making anyone happy. Have you heard of the book Stepmonster? You might read it and give it to your significant other, or suggest counseling to help him realize what this could cause down the road. If he is unwillint to meet your boundary of not being ok with this behavior and the message that it sends, then you might need to consider if participating in this is something you really want to do.

No, he should not be giving her extra money, helping her around the house, helping her with car stuff, etc. They are not married anymore, his loyalty should lie with you. Do not put up with this, make it very clear. Otherwise you will become very resentful. Stepmoms or girlfriends are often treated like second class citizens, or not part of the core family. You deserve someone to take care of you.

Sorry but no. It will create more confusion for his son. If his marriage is ended, it ends. Son comes to you guys on his days and stays with her when it's not. This whole family bull cr*p for the children boils my p*ss. As for paying for things for her, that's what he provides money for his kids for- or buys stuff his kid needs. There's a line. Paying for car stuff is not for his kid. That's for her. My OH does it too. She's recently given him a letter of tax overpayments to her for nearly 3k. He wouldn't have told me about it if I didn't see it. Says he's not going to pay it but I think he already has. And if I find out he has, out marriage will not be happening. I already told him she sorts it out herself and if that means the kids come to us every night for tea to make sure they don't suffer then that's what will happen but SHE is paying her own debts. Personally I think it went in one ear and out the other but unfortunately unless I snoop through his bank statements I will never find out if he's paid it. 

There is no family that includes your SO, his X, and their kid.  That family failed and ceased to exist when she bacame his X.

This facade is nothing more than an attempt to lie to this kid, or worse, a facade for your SO to remain in an intimate guilt driven relationship with his X.

This screwed up failed family fantasy will mess this kid up until he is F'd up beyond salvage if this does not end now.

Dad needs to model a confident man, husband, and father for his child instead of creating an existance of bullshit for his kid.

More importantly, he needs to live a true commitment to you. If he won't, boot him and his failed family fantasy from your life.

Good luck.

He told me he needed to "help" her for the kids.  He said she wasn't the best mom, but needed to be in their life so the best thing he could do for "them" was to support her.  He also claimed that he was setting a good example and showing his children a good man.  He was supposedly doing this so his kids could see that they could still "work together."  She would call, and he would walk outside.  She would text at weird times.  He offered to let her stay at his house during a hurricane.  One night, his daughter told me that dad tucked her in last night (when she was with her mom!!)  I confronted him, and he said his daughter was confused and imagining things.  Then.....I finally gained access to his phone (he guarded it like a hawk).  I watched him put the password in and remembered it.  Low and behold....texts saying I love you and will always miss you.  Him telling her he just wanted his family back together again.  Him saying all he wants is to hold her.  AND dirty pictures they both sent of one another back and forth.  It was going on the entire time we were together.  All the times we fought and argued and he said I was crazy.  Do NOT trust him.  When it's over, it's over.  That is it.  It's NOT over.  I'm so sorry you're going through this.  Please move on for your own sake and your own happiness.  Life is too short and you don't need to play second fiddle to an ex.