How do I motivate my 4th grader to get better grades?

Dear Dan,

I have a fourth grader who, until recently, has been a straight A student. Last year he learned how to do short linear equations, and he enjoyed it! In fact, he has always excelled at math. However, this year I have seen nothing but low B’s, C’s and even a D from him. This this isn’t even the hard stuff! It is simply addition, subtraction and multiplication.

We have now had two very serious discussions regarding this problem. After the first I offered an incentive for good grades. That obviously failed and I have now resulted to punishment for the poor grades. Yet I am still not sure I am getting through to him. This is the first time I have encountered this problem and I have emptied my parenting toolbox. I am desperate for a solution and determined to correct the behavior. Do you have any suggestions?

Thanks, Desperate & Determined