Dogs have trouble learning distance and duration at the same time. Work on the distance first, which will build in duration by default. Put him in the down, take one step back, one step forward and reward but don't click (clicking, or marking the behavior signifies the END of the behavior, which is not what you want in this case). Make it fast, like you are attached to a bungee cord. When the dog can stay down for 10 times at one step away, consistently, then move two steps, then three steps, and so on. Make sure you use your release command so he/she knows when it's okay to get back up.