Swamped

You can't do a whole lot here except bluff. Although, you do have a hostage. You just lack options for doing much about said hostage.

"I should think it would disrupt the business model if you had to find another chemist," you note, backing away slightly.

"That might be a more effective threat if you had a weapon. At your age, I doubt you could hurt him that badly with your bare hands."

"I'm a wizard," you reply.

That makes him pause. Which is enough for you to adjust your position and get the chemist in front of you. If nothing else, you can make his shot riskier.

"You're bluffing," he says finally. "If you had magic, you could have done something by now."

He doesn't sound entirely convinced. Maybe you could pull a little trick to scare him off.

Then again, you do have a decent negotiating position right now. Perhaps you just need to convince him you're not a threat.