M.L.
Children learn by imitation! Your son has been imitating his parents ever since he met them. He's been imitating everyone around him. Wait until he's a teenager - then you'll see imitation!
Five is the oldest your son has ever been, but it's still pretty young. The question is whether his older sister and his younger cousin are giving him good things to imitate, If they're being good role models, you all don't need to worry too much.
Some children have personalities that make them look to others for leadership for a longer time. However, as they get older, they *do* learn to think and act on their own, and often turn out to be better adult leaders than the ones who always led the crowd as kids.