What sort of stage are you at if you're working on his ward? Are you a work experience school student or med student? If the former I'd ask more general questions eg good and bad things about his job, his training, hours, time for family and social life, administrators/ managers interfering in his job, has it changed much over the past few years, how much research he has to do/ wants to do those sorts of things.
I'd probably steer clear of asking him about his salary, you can look up NHS salaries anyway.