She won't suffocate him, but if he doesn't want her in there, keep her out. My 7.5yr old LOVES when our cats sleep on his bed (they are rotaters at night ,on our bed for a couple hours, his bed for a couple hours, then back to our bed... etc) and they always move together (we have two male cats). Our 4yr old, however, is less than pleased when he finds them on his bed, and they've learned to just stay out of his room. He doens't like them laying on his blankie, or on his stuffed animals, or on his feet. He's a mover at night too, and doesn't like being restricted. :)
If he is ok with the cat sleeping with him, he will get used to her movements. I dont' notice when our cats get up and move beds, neither does our 7yr old, we are used to them moving. If he doesn't want her there, then just close his door nad put up the baby monitor again.