I agree with Sam, but it might also work to give the student long-term or short-term projects that might motivate him. He could work on the project whenever he's done with his homework and he could show the class his results. Project assignments make it easier for the teacher too, so that s/he is not trying to find something for this student every day. How about the four 4's problem? (Use only four 4's and mathematical symbols to represent the numbers 1 to 100. e.g., 8 = (4+4) x 4/4. He could use exponents, square roots, whatever fancy stuff he has learned from dad, as long as the only numbers used are 4's.) Try to find projects that deal with the same math topics as the unit the class is working on.
Also, see Ask an Author for the recent response on how to challenge the gifted child.
It is great that this father likes to do math with his son, but I would ask him to re-think what kinds of math to do at home. If he continues year after year to teach his son the content in the next grade level, his son will continue to be bored in class-- you can only expect so much from a teacher to modify things for one child. Maybe the father doesn't know what else to do with his son, and would appreciate names of resources he could use that would push his son horizontally, not vertically -- that wouldn't just teach his son the content in the next grade level. Again, see Ask an Author-- both the gifted child response and the "more homework" response.