If you are having difficulty removing the Tuition Credits transferred to you from your son, try removing him as a dependant altogether.
Proceed to Deductions > Claims For Dependants > Claims For Dependants Intro. While on the Claims For Dependants page, make sure you have answered Yes to Would you like to review or claim any of these credits?, click on Enter Credit Info next to your son's name.
Then continue through to the page where you have entered the Tuition Information, and ensure it has been deleted. You won't be able to continue from here if the page has nothing on it, so go back. Then if that Tuition is still there, return to My Info, Dependants Information and "Delete" the dependant altogether.