You need to get him up that's the difficulty part, a feather on a Rope is a nice tool to get him playing as example, so he would tries to get up.
if he start to stand up, give him a reward. every times he does it give him a reward. IF you would do that with a "clicker" you could conditioning him for it aswell * just a idea * search for the internet for clicker training if you don't knew that kind of stuff.
my cats are knocking on the door when they want in and out.... need a Bell button for them on the ground