Teach him to use language precisely. The idea that words can have hard, exact meanings is fundamental to mathematical thinking, and will also serve him well in most other subjects.
To give a few examples, in a few years, you'll have to discuss with him whether a square is a rectangle, or whether a right-angled diamond is square (and unfortunately, your child's teachers may have a different answer).
But my advice is about
And I mean all language, not just mathemtical mathematical language. Being clear and precise in one's language fosters a mathematical attitude by developing one's ability to make and reason about fine distinctions.