How do the 529 distributions in 2017 compare to the school's Cost of Attendance (CoA) figures for academic periods in 2017, based on your son's living situation?
Aside from textbooks, required course materials and computer equipment, did the 529 2017 distributions equal the 2017 CoA as published by the school for your son's living situation? If so, then you're done, and any further distributions will be assessed tax (and likely the 10% additional tax as well) on the earnings portion of a distribution, because there are no more qualified expenses to pay. If the 2017 529 distributions fell short of the 2017 CoA figures for your son's living situation, why did that happen?