my can't jump

Why does he have to jump? Maybe he just doesn't want to. Dogs are different like people are different from each other. He may just think it's too much effort. Maybe he is scared. I don't really see any reason to worry or to force him to jump. It's not an essential survival skill for a dog to have.

Why do you want him to jump? Is he old? Is he hurt perhaps? Can he walk and run? If there seems to be a real problem, please go to the vet and have him checked out so he isn't in pain.