Give him time, they expand their action radius gradually. Maybe leave a treat on the first step down now and then, when it's gone, next step. Maybe showing him what the stairs are for might help, by going up and down when he can see, or even sit there for a while.
One thing, rabbits like to have good traction, for mine are raw, rough wood stairs and carpeted stairs no problem at all. They don't mind slick flooring either, but I prefer they have a good grip when they leave ground level.
When I got Herr Hase getting out of the hutch was a new concept for him, he had no idea what to do with the ramp - had to send some kits to show him the ropes