Do you think if ALL of you go down the stairs and call for him that he'd come?
We have really steep stairs at my mom's house and when my sister first got her greyhound, Maggie, she didn't like the stairs either. She could go up, but refused to go down and we'd have to carry her down. Eventually, she just started doing it. Since my sister's bedroom was upstairs and that's where the dogs slept she didn't have much of a choice.
How bout if you (well not YOU, but someone) carry him down, see if he'll go up by himself? And just each time carry him down and maybe he'll learn it's not a bad thing. I don't know, I'm just throwing out suggestions.
Good luck!