I am so sorry that their behavior is appalling. A ton of people said that couples' and individual counselling is the first step. It's great if he's willing to acknowledge and explore the problem.

I'm going to suggest something that's a little invasive and prolly none of my business. I strongly suggest you be diligent and careful about not having kids with this dude for the present. If things go further south, and you have to deal with custody battles and/or all the potential family drama that would likely come with bringing a child into that fucked up family, it's going to be incredibly hard. You should hold off. If your original plan was to try for kids, you should simmer that for a while until you see concrete, actual change in his handling of this situation.

His family insulted yours, insulted you, and completely disregarded his weakass attempt to draw a line between them and you. You're hurt and sad. You have been deprived joy. Fuck them and fuck him. And fuck that he is sneaking behind your back to talk to them, especially about you and your plans. Nope. You might not be able to control his relationship with them, but you sure as fuck can let him know that you'll be controlling your relationship to them, and you want none. That includes him keeping your fuckin name out of his mouth if he's talking to them.

Make sure you keep telling him how you feel, too, so there's no question of holidays together, or vacations, or whatever, and make sure he knows why. You should be real open about the problem. When something happens, you give him those specific examples. Maybe even write them down somewhere. Build your case. You'll either have a list of reasons to do therapy or a list of reasons to get the fuck out.

This sucks, dude. But his family is a big old red flag, and another is how he's handling the situation. Good luck to you, I'm so sorry you're dealing with this.