Tell him to exhale in the direction of the class, and say he just exhaled 100 atoms of some trace gas. Xenon or krypton, I don't know. Or teach him scientific notation, and have him write 100 * 10^20 or some big number on the board - the number of atoms he just exhaled.
(Hey, when I was in kindergarten, I tried (unsuccessfully) to explain infinity to someone who thought 100 was the biggest number. Kids love big numbers.

)
Teach him binary, and have him write the numbers from 1-100 in binary.