Verified answer

If he knows how to multiply, you can teach them the trick of moving the decimal depending on how many zeros there are. Then you can prove to them by solving the math problem and showing every detail.

It's just a question of moving the decimal point and adding or removing zeroes. The metric system is great because it's all based on tens and it's very logical and simple.

Now if I had to teach a 3rd grader that crazy American system, I'd have no idea how to do it. Good luck!

get a big ruler and show him/her