I have 3 dogs who love our beds, also! I bought them each a cooloroo dog bed, and they fight over them. Here is the link:
http://www.google.com/search?client=...UTF-8&oe=UTF-8
They are inexpensive and easy to set up. The dogs like them because they are off the ground.
I would set it up, and put one of his tshirts or something with your son's smell on the dog bed, then he will "know" it's his! Then, I would shut the bedroom door.
Good luck!