[Jemma]

To the OP, I would definitely try pee pads upstairs. If he/she feels like he has to go in a hurry, there is always a pee pad available for him. Perhaps you can do as a poster above stated. Put him on the leash for a lttle while. Pee pads have a certain scent, people can't smell it, that brings the dog to the pad and allows them to eliminate ther.
Good luck, I ave a little Shorkie such as yourself. She is 8 months now. She goes on her pads. But she won't poo and pee on the same pad, grrrr. So I have 2 pads where she now goes. And in seperate areas in the house. Better that than all over the house.