Benjy wasn't about to arrest a kid. It wasn't something he wanted to do even a little bit. "Was it Slughorn's potion?" Why bother asking, he already knew the answer. Of course it was. And Michael had some semblance of his magic back, even if it was wild and uncontrollable.
He wanted to question that, how it felt, if Michael could do anything except accidental magic. Anything. But he wasn't a healer and he definitely wasn't Gabe. He wouldn't have any idea what to make of his answers except to take some glimmer of hope for the kids who had lost their magic.
"Well, you're going to go to Madame Pomfrey and you're going to get tested." He paused for a moment. "Can you do anything besides the waves you can't control? Have you done anything you can control?"