Hi Overstayer,
The George Robson, Painter, in 1841 census in Hexham, has a Mary Robson, age 20, probably a daughter, living with him, but no William.
Which census do you have William, born Hexham, in? Can you give the reference info, or give some info on where to find him, wife, children, occupation etc.
I presume you have a copy of his Marriage Cert, which is how you know his father is George, a Painter?
With a bit more info, maybe we can find William, or some of his siblings baptisms, or Georges marriage, might give more clues.
Alan.