So my boyfriend and I had this really great idea for halloween. We're going to be the yellowbrick roaders from the wizard of oz. He will be the lion, I'm the scarecrow, my son is the tin man, and my daughter is dorothy. When we brought it up to the kids they were really excited about it and we started talking about how we'd make our costumes. We're gonna make really nice costumes so we can march in the parade, and we've already started them.
My son told his dad about it, and his dad told him that halloween is only for kids and adults aren't supposed to dress up, and if he goes with him for halloween he gets to be whatever he wants. So now he wants to be scooby doo and go with his dad. I'm totally annoyed about this, because he was excited about our original plans, all of us were. Any ideas on what I can do? Now my son feels torn between two parents. Should I just give up on the plans and let him be scooby doo? Should I let him go with his dad and find a new person to be the tin man? Should I try to talk him into it and talk to his dad about it?

Well maybe he can be tin man with you and scooby doo with his dad and you can split the night. And each be responsible for one costume. That seems like a good compromise.
I think that's what we'll probably do. But we've been alternating halloweens, and his dad had him last year.
Had you and his dad previously worked out Halloween plans (i.e. who your son was going to spend Halloween with?) If not, I think that adcaela's suggestion of splitting the night makes sense.
Also, what is your son's relationship with his dad like? What is his relationship with your boyfriend? What about your relationship with his dad - are you on good terms at all?
we're on okay terms outwardly, but he undermines me constantly and is very competitive when it comes to our son and his love for us. He and my boyfriend seem to get along, but he's definitely jealous.