How do I tell him it's no going to be us?

Me and my ' friend ' tre are going on a ' date ' tomorrow and me and my other friend cooper are the ones who planned to go to the state park together but I just invited tre to come along and he thinks it just going to be me and him and idk how to tell tre that cooper is coming too because I don't want to tell cooper he can't come because it was our plan first but I don't want to bust tre's bubble and tell him it isn't going to be just me and him but I know I have to so how do I do it ?!