Your son was perfectly aware of the partiality Drona cherished for the Pandavas; so to make Drona strictly redeem his promise, Duryodhana gave publicity to that counsel.
पाण्डवेयेषु साक्षेपं द्रोणं जानाति ते सुतः। तत: प्रतिज्ञास्थर्यार्थं स मन्त्रो बहुलीकृतः॥