How can I tell my child that there is not enough money to attend the top college?

Work together. The issue is attending college and how he is going to get there. There are numerous grants, scholarships and loans that make education attainable. What does "top" college mean? Kids can have a "top" education if they make the most of the opportunities presented to them no matter where they go. Community college is a great staring point, making credits affordable and transferable to 4 year u´s.
Be honest. There are many ways to attend college on scholarships, grants and student loans. My parents had no money for me to attend college; but i've put myself through 5 years of undergrad, 2 years of grad school and almost 4 years of med school. Where there is a will, there is a way! my 1st year of college was paid with 8 small scholarships i got my local community organizations + a federal grant.