Despite how pissed off Jason was becoming, he admitted this kid was good. Stubborn. Jason would keep playing this game as long as it took.
"Don't you have better things to do than trot around in stolen clothes?"
Thievery was just an assumption, but also the most probable answer. Bruce wouldn't have given it to him. Then again, there was that other Robin. Completely new costume, but he had still been given the role. Rather than do what needed to be done, Bruce just kept picking up kids and putting them in costumes. Jason tensed up a bit, visibly.