Why is he on the hundred dollar bill?

By "he" I assume you mean Benjamin Franklin!

Ben Franklin was one of the Founding Fathers of the U.S. and arguably one of the most accomplished and intelligent men in history. He was a scientist, writer, publisher, composer, politician, and philosopher, as well as being a major voice in helping to write the Constitution - just to name a few talents. Any one of those alone should rate being on a banknote, and he did them all.