From your post, it sounds to me like food would be the best way to get him up. How about putting his food a few steps up from the bottom and see what happens. If he is happy to come and eat it, you could start feeding him further and further up the stairs.

My cats were feral when they appeared in the garden and I know all about how long it can take to get them inside, but Macduff obviously trusts you already, so hopefully it will be a bit easier to get him to come upstairs.