My bossy elder daughter had to be told many times that I was the "Queen" for now and when she grew up she would get to be "Queen," but for now she had to accept her place in the world as a kid which can be a lot of fun. Now as an 11 year old she doesn't test me anymore around authority and is happily a kid. In fact now she doesn't want to grow up because it means a lot of responsibility. (Another challenge in the opposite direction, OOPS) She still tries to boss her sister, but not NEARLY as much as I have taught her that she is her sister's friend not boss.
As far as the mirroring, it's important to hear them, mirror them, AND say how things are. Kids want to be heard (don't we all) and they also need to be guided towards the positive truth rather than focusing on the negative.
I would be curious why he wants to go back to pre-school. What did he get out of that experience. You may be surprised at his answer.
It takes time and, yes, patience. Sometimes I think parents nowadays are too afraid to assert their authority. I'm not wholly on the Dr. Phil bandwagon of parenting, but kids need solid boundaries to grow up feeling secure within themselves. One of my good friends says, "It's a kid's job to push boundaries and a parent's job to reinforce them."
And when you really need to, give yourself a time out. LOL