If you don't use that knowledge for your profession, then yes you will forget it. However, if you don't use it for your profession, it doesn't mean learning it was useless. I would be more worried if he still sucked at math after preparing for the test for a semester or two.
I have a degree in Chemistry and yes, that knowledge is slowly slipping out of my mind. However, the skills I learned while learning chemistry - logical thinking, ability to memorize and recall, building a mental model of abstract concepts - these stay with me.