Please don't have him declawed, especially if he goes outside!

OK now that I have said that, I would also recommend Soft Claws, and also trimming his nails often. I trim Sophie and Tito's nails down at least once every ten days. I keep them as short and as blunt as possible, so it's harder for them to do damage. When I do catch them scratching things they're not supposed to, I nail them in the butt with a water gun, but since Barclay is scratching at night, that may not be helpful for you. What I would suggest is:

* Soft Claws OR

* frequent nail trimming
* a new sisal scratching surface placed right next to the stairs - a rope, etc. (I know you already have a post there, but keep trying )
* water gun therapy
* covering the stairs with carpet?
* keeping him in another part of the house at night, if possible
*A cat tree or something more fun for him to play/climb/scratch on at night

I don't know exactly how he's scratching, but some cats like horizontal or slanted scratching surfaces as opposed to vertical. Maybe that's why he likes the stairs? I have a couple cheap horizontal sctratching surfaces that my cats like, maybe he would be interested in something like that.

These are horizontal scratching pads I have.

nail clippers

Soft Claws

Good luck!