sweetsomethings sorry things are tough but great you said I'm just going to have to sit him down and say i should never had said he couldn't have his party.That even adults can say silly things they don't mean.
That is exactly what I would do.
You are right to work out you do not have to see this through and you can give him his party. You can engage in a conversation with him about his behaviour and work out what is going on and why he is being so difficult.
Behaviour is sometimes/often/always - * (delete as you like) communication.
If he is not bothered about the party could he be unhappy at school? Maybe he is being bullied or fallen out with is mates? Or just hyped up about Christmas, as others have said.
He is only 5, if this expression is new (see if I care or whatever) maybe he is trying to communicate with you that things are not well.
As adults if we say something we regret we do not feel the need to follow through especially if the original thing said was unfair or silly or whatever? We say, I was a bit hasty there lets work it out.
Agree with makapakasdirtysponge that How to talk so children will listen and listen so children will talk' is a brilliant book.
Good luck.