Hello!
Your story sounds exactly like the tutor that we have known and used for years. When he was very young, he was identified and pulled aside by the Stanford professor who ultimately created Stanford's Education Program for Gifted Youth (which then birthed Stanford's Online High School). This tutor specializes in working with gifted students.
I could imagine that if you had your 3-year-old having an online lesson with that kind of a tutor, under the distant supervision of your nanny, she then could focus her attention on the 16-month-old. I would not worry too much about socializing him with other 3-year-olds since then you don't run the risk of your intention backfiring where he learns typical 3-year-old behavior and becomes less interested in learning geography, etc. and picks up bad habits.
I am relatively new to the forum so I do not know if this group exchanges information but I would be glad to give you his contact information if that would be helpful.
