No offense, but I'm a bit confused.
Because a birthday party might be too expensive, he was just going to take him to Universal Studios which you could not afford to participate in .
So....he made a different arrangement and even though it wasn't said in the nicest way, you were invited if you want to come.
From the outside looking in, I'm not sure why you are upset. It seems to me your ex tried to do something that you could be included in with no expense to you. I'm not understanding how that HAS to be construed as going behind your back unless of course you wanted to be included in all the planning.
I wouldn't say a word at all to the new fiance about it in terms of your feelings being hurt.
My ex and I got along so horribly that we had separate birthday parties for our son. I tried to have us all together and it just created more pressure than enjoyment for our child so we did different celebrations.
But, if you don't want to be left out or feel left out, graciously accept the invitation, as wimpy as it sounded, and try to have a really nice time.
The day is for your child. Hopefully, the only thing HE will get out of it is that he has lots of people who love him.
If you don't think you can go and everyone get along, graciously decline, but they did extend the offer for you to go. You said yourself that he's trying to take into account you couldn't afford Universal Studios, etc.
You know, not all women are evil. Perhaps it's the fiance that wanted you to be included.
I think you should go, IF you feel comfortable with it, be yourself, interact as a part of your son's life as well as his extended family and do what you can to be part of a wonderful birthday for him.
I'd like to add a side note....after 14 years of divorce, my ex-husband and I have had a full turn around. We celebrate all holidays together, etc. Our son is 16 and my daughter is 25 and she gave us a grandchild. That pretty much threw the division of things out the window. It's worked out quite nicely, much to my surprise and joy, to be honest.
Never thought I'd say that in a million years, but it's pretty great.
Go to the party, have a great time, and do it for your son.
Just my opinion.
Best wishes.