Sit on the dog bed yourself and see if it is actually comfortable. He will probably instinctively rock back into a sit to reach for the cheese. When he sits, give him the cheese and praise him. Do this many times in a consistent manner and he will learn to sit on command. Once he learns, you won't really need cheese anymore.
Some trainers say food rewards are bad.
Both methods can work so why should I push my dog friend around if I can be nice. Next, put a piece of cheese on the floor and pick it up again. Do not play mind games by encouraging the dog to get the cheese then grabbing it. Just try to be clear about putting it out or picking it up. The dog will probably start to watch and wait just a second to see if you will take it back or leave it for him.