Counting challenges

On the way home from school today Jacob nonchalantly announced that he was going to count "in trinary." Then he did. 1, 2, 10, 11, 12... What?!? Knowing that Craig had taught him the binary system I asked him where he learned trinary (is that even a correct term for Base 3??)--he told me he figured it out himself (knowing what he already knew about binary numbers... he then went on to count in Base 4 next...) [Craig later confirmed that he has not taught Jacob any Base 3 or 4 etc...] Anyway, when we arrived at home Jacob ran for the paper and pencil and wrote down his counting! Almost 4 years old...