Cats have different preferences for scratching surfaces. Try different ones and find out what his preference is. Rubbing catnip on it will encourage his scratching, as well as your praise when he scratches where you want him. My boy likes a sissal covered scratch post and a door mat that has small tight stiff loops I got at a hardware store. My girl's favourite is a cardboard egg carton on the floor and the door mat. Cover your sofa with a sheet until your cat has got a well established place to scratch. Good luck!
"We humans are indeed fortunate if we happen to be chosen to be owned by a cat." -- Anonymous