My coworker (not my boss) asks me to do things that aren’t necessary or in my job description to “stay busy” and to build up “my work ethic” because I’m young and will be here for 20+ years

He is older and has a few years left and while he is right to some degree I always do what I am asked by my actual boss and I don’t want to argue with this guy but it’s starting to get on my nerves how would you handle the situation