I really don't think there is any 'norm' and I definately wouldn't compare your child to others.... They are each as different as all the adults you know.... People sleep different amounts, eat different amounts, grow at different rates, etc Every kid has there own needs and I think it depends on how tired they are from the activities of that day or from the day or night before, how they are feeling, what the environment is (are they home, out, in the car, etc). If you did the EXACT same schedule, activities, meals, weather, etc then I would say that an average 21 month old naps a total of 1-2 hours in daytime rest.... whether all in one afternoon nap or broken between a couple cat naps, but again it's so variable and every day could be more or less.
I would stress as little as possible about something that occurs naturally as needed. It can't really be forced but you can set up the environment to be condusive to rest.... Calm, quiet, comforting. He will rest when tired if allowed. The more physical and mental activity he has, the more likely to nap often out of need, and the healthier overall he will be too!
Best wishes, don't stress, every kid has different needs and as long as he is given the opportunity then he will take whatever naps he needs....