It IS seperation anxiety but the fact that he is urinating on your bed shows lack of respect for you as alpha. he is showing you by breaking down the barrier thathe has a complete disregard for your authority. Unfortunalty this type of breed will take a inch of your weakness and run with it. I have seen this happen all to much with this breed....
you get what you put into these dogs. Too many people turn them over to shelters because they "are too stubborn, physically too strong, have too much energy, or didnt turn out to be as esay to train as the lab I had last"
First, you need to make sure your Staffie is getting plenty of exersice before he is left at home alone to be bored outta his mind. Too many people wake their dogs from a full nights sleep and leave them at home to go to work in the morning.......recipe for disaster.
Second, you need to beef up on his obediance. He should feel that he has "no right" to do the things he does. He has no respect for you.
Lastly, he may need to be crated to show that his behavior is unaaceptable, nothing you do to repriamnd him when you come home tells him what he is doing wrong, its to late, he already forgot about it.
End note: I'm not implying you do so BUT force will NEVER work with this breed, if anything he will become more stubborn. Patience and respect for his nature is the key to success here. Good luck and keep us posted!