Great idea GeorgeofNE on the cardboard.
I was going to second, or third, the whole stand behind him and move his paws one-at-a-time till he gets the motion down. Down was much scarier for our dogs and for me, so that is great that he's able to go down without attempting to jump the entire lot.
Took Kingsley about a week to master up and a month to get down (but we also have scary curved stairs) while Ruby, our second conquered both within days since she is much more outgoing and had Kingsly to watch.