My german shepherd doesn't want to walk at all?

I have a German shepherd.He is 3.5 months old and i have kinda awalking problem with him.He doesnt wants to walk at all when he is on leash.Without leash he walks perfectly near me and follows me everywhere.On leash he just sits and doesnt want to move at all.Without leash he sometimes sits but when i go 5 meters away he runs after me and then he continues along me normally.