Can you first get him excited (ie. tug, OB with food) around the stairs first then run up 1 stair step to do a tug/OB there, or lure with food? If he gets comfortable and excited around the stair it might be easier to get him onto the stair. Or, feed his normal meals on the stair case and up it slowly each time?
If there's a dog he likes to play with, can you borrow that dog and play your way up the stair?
I'd not make the stair an event in itself but incorporate it into something he likes to do and very little at a time so he's not so focused on his fear but the fun/motivator.
Would he follow you if you go up the stair and disappears - would he come looking for you? Have someone hold him at the bottom and release the leash if he wants to come looking for you?
If he's into flirt pole maybe you can use that to entice him?