Weight training is important once you get older. You lose muscle more easily and it is harder to gain it. Like everything else physical, when you're older you have to be more careful about starting a new activity. If the gym has trainers they may be able to help him get started with easier moves.
"bad back" is pretty generic, but I know that working on my core strength has helped my back.. if I keep my core strong then I don't pull muscles in my back as often.