Unfortunately, they also completely lack insight, so their sole goal is imitation regardless of any consequences.

I suggest you take your son to the playground, to the library story hour, to a drop-in gymnastics class. Provide him with opportunities to interact with his peers; when he meets kids his own age, you'll find that he'll enjoy their company and he'll learn from them.

You might also enlist your siblings in the project; encourage them to help you with your 3-year-old. They know that he loves to imitate them, so challenge them to be good role models for their nephew.