He would have had a hell of a time getting him down from the Astronomy Tower undetected with all the OOTP and DA members at the foot of the stairs.
But he had another opportunity near the gate when he revealed he was the HBP and still didn't kill or kidnap Harry.
No, not then--when Harry had chased him out of the castle and they were nearing the boundary of Hogwarts and the road to Hogsmeade, where the Death Eaters all headed so they could apparate. He had the perfect opportunity! He even stopped while Harry was on the ground, screaming "I am not a coward!" at him. He could have grabbed him then, but he didn't.