1) Tom is the fastest boy in the world.
2) Tom is faster than any other boy in the world.
3) No other boy in the world is faster than Tom.
4) No other boy in the world is as fast as Tom.
We say "no other boy" because this means all of the other boys. You will hear people say "No other boys in the world are faster than Tom", but it's incorrect. Only ONE boy can be the fastest in the world, and the use of "other boys" implies a group.