Fiancé wants ex-wife at our wedding.

I'd use a lot of caution here. I'm not sure why your FI is sort of emotionally strong arming you by threatening to "tell the truth". Being in a marriage means getting on the same page and presenting a united front to the world. I don't love that he's already holding you responsible when he doesn't get what he wants.

In this case you have to set the boundary I am uncomfortable having your ex wife at our wedding and feel like it's very inappropriate. This is not an unreasonable ask in my opinion and it is extremely immature and disrespectful for him to throw you under the bus, I would think most people do not feel comfortable having their spouses former lovers especially ex spouses at their wedding. If he can not be a mature adult here , respect your feelings, and present a united front on this decision that's a huge red flag. Best of luck to you

Without any kind of relationship between you two, she should not be there. Period.

That's a conversation that needs to be squared, and for your fiance to understand. His goal in this situation should simply be to not have you feel uncomfortable on your wedding day, whether that is to spend the time leading up to the wedding to build some semblance of a relationship between you and his ex, or simply to not have her there.

To overlook this is incredibly obtuse.

There's always the chance that she would say no, too. We asked DH's ex-gf because they have a daughter together, and a lot of ex-GF's family was going to be there because we are still close with them. She declined, but I would have felt more weird NOT asking her. If we weren't close to her family, it may be different, though

Let your son invite a couple of his friends to the wedding to allow him to feel at home - maybe they could be page boys or ushers together. He may be anxious about what this new step means for his place in your "new" family. You should make him as welcome, special and involved and relaxed on the day as possible to set the tone for the future. The only answer he should get as to why his mum is not invited is that it is a special day for just you both and him. But that you can have other special days that include his mum, like his birthdays and special school events etc... Stepmum, please try to feel secure in your relationship regardless of how your stepson's mom behaves and rise above the urge to hate on her. A detached neutrality is best. Kids pick up on everything, so focus on building your relationship with your stepson and his dad not hating the ex. your fiance's plan on what to tell his son is traumatising. A child should never be aware of adult conflict and knowing you have any animosity towards his mum is only going to create an emotional weight for a kid to carry and create a barrier been you and him having a happy loving relationship.