That's not a bad thing. You two have a secret between you. Helps the bonding, Wet. Not that you want to encourage him to keep secrets per se, but some are going to be ok and even helpful. It's something the two of you share. Besides, he was going to see that movie with or without you at some point Better to be with his dad and learn how to deal than to figure it out with his friends. Seriously. Just don't make that a habit, right?
As for the moving. I get what you're saying. I feel it. But honestly, he is affected by his mom's choices as is the whole family. Some things you can protect him from and others you cannot. What you can control is being there for him regardless of what life throws at him. And he can always come live with the stable parent - you. All of that can happen without you moving in with her.
Life is a funny thing, Wet. Your job is to teach him to be a functioning member of society. This is something that happens in our society. You can teach him how to deal with it by showing him. By being the stable parent. By being his dad.
He may not like all of it, but that's how life can be. He'll need to learn (from his dad) how to deal with life.
AJ
"Our lives begin to end the day we become silent about things that matter" MLK Put the glass down... "Yesterday I was clever so I wanted to change the world Today I am wise, so I am changing myself."
Better to be with his dad and learn how to deal than to figure it out with his friends. Seriously. Just don't make that a habit, right? 
