Climbing stairs, though far from easy from a dog’s point of view, is still much simpler than going down them. Although going up relies on upper body strength, going down is more a matter of balance. You should train him how to sit or stay first. do the same thing with the treat. show him by positioning his body how to sit. put you hand on the ground and push his body down. once he does give him a treat. for him to stay, show him the treat. then slowly walk away. say stay. if he does, give him the treat. When you get to your house and he approaches the stairs, tell him to sit or stay. if he does, give him a treat. and then walk upstairs allowing to see another one. once he does, give him another one. Eventually, stop giving him a treat every time he sits down but instead when he goes up the stairs.