I think first you will have to make sure that he has a clear understanding of integers and what numbers are integers, with that clear you can explain that a rational number is an integer that can be expressed or written in the form of a fraction both the numerator and the denominator are integers. The rule you have to keep in mind is to make sure you explain the b cant never be zero because you cant divide by zero . so you cant have a zero in the denominator .and an irrational number is a number that you can express or write as a fraction using integers . a great example would be pi .. hope this help a bit