Teeth do get yellower as they get older, but as randomwiktor said, give him somehting hard to chew on, you can buy blocks of flavoured wood, like strawberries and carrots etc, t entice them to chew it.
I don't know if its true for hamsters, but rats chatter their teeth when they are contented. How does he seem when he's on the keyboard, as in, happy, or scared or what?