Squeaking Hardwood Foyer?

Does anyone know what to do about a hardwood foyer floor that squeaks. The contractor did not put enough cement down when the boards were glued and it is squeaking badly in parts. We asked him to come over and check it out; his reply was: "can't do anything about it now except to tear it all up and start over." This would be too costly. Does anyone know if this can be solved?