[Marilize]

I agree with everything the other post said. To get him to realize that you are the leader, you need to control the resources. For example, do you free feed him? If so, change to scheduled meal times. Before you put down his food, he has to perform a command. (I have taught Cash to sit and stay until I say the release word and only then can he get his food.)
Also, after 15 minutes, take the food away and give it again at the next meal time. I promise you, he will only go hungry once.

Other leadership tips: Always have your head higher than his, claim the best spot on the couch for yourself and always enter a door before he does. This firmly, but subtly lets him know that you are the leader and that he is second-in-command.
As to the separation anxiety, ignore him for 10 to 20 minutes before you leave and after you get home. I know it is difficult, but it works. Leave a radio on so that he doesn't feel alone and give him some intelligence/activity toys, like a treat ball or a kong. Only greet him when he has calmed down - if he goes crazy like mine do.
When you get home and he has done something bad, don't yell at him or show anger. Just put him in a safe place (out of sight) and clean up what he did.
Remember that the leader is calm, confident and assertive. Keep your cool. When he is good, acknowledge those behaviors and when he is bad, simply ignore him.

Good luck. Let us know how it turns out.