This is pure speculation, but my guess is that the father feels that learning how to answer multiple choice questions about math is not the same as learning math, and that the free-form questions are, at least in some sense, closer. Hypothetically, if one knows how to write out full, complete solutions to every problem, one should also be able to get the answer on multiple choice without any issues. (There is also a peripheral benefit to this in that the kid will do better in future mathematics, after the test is over.) He may not realize that you are teaching math over test method, and you reassurance that this is the case could be exactly what is needed to appease him.
how to answer multiple choice questions? | Yahoo Answers