Lego is a great tool for this. I also taught prime numbers by handing out (round) 1x1 pieces and told him to figure out which ones could be made into squares.

Lego is a great tool for this. I also taught prime numbers by handing out (round) 1x1 pieces and told him to figure out which ones could be made into squares. The teacher was surprised when - in the middle of a more convoluted explanation for primes - he just said "primes are numbers that can't be arranged as rectangles".