Swamped

Your best chance is to figure out how the spell actually works. Checking the ether only tells you that it's light-based, but what does light have to do with holding still?

An illusion, perhaps. Something only he can see, and seeing it compels him to stay still. His vision seemed to be especially good, which might even make the spell more effective.

Sound opposes light, but both types of spell are based on perception. The blind can't be affected by light magic and the deaf can't be affected by sound magic.

For just a moment, it crosses your mind that you might be able to break it by blinding him, but that's obviously too cruel. You can't even be entirely sure it would work - he might have to realize he's blind for it to actually affect his perception. It's certainly not a theory you have any desire to test.

What if you thought about it outside of magic for a moment? Light and sound oppose each other inherently, but part of the reason why is that they affect different senses, and so sound magic will emphasize the sense of hearing and distract from the sense of sight. So perhaps a very strong smell would do the trick. You're a bit skeptical - it didn't work with touch, or he would have reacted when you shook him - but it also wouldn't be that hard to test if you had a strong enough smell. But where would you...

Fish. Mortimer mentioned the smell of fish earlier.

"I've got an idea, but I can't guarantee it. We need to take him near a fishmonger's. Whichever one's closest."

Black nods, and points his arm in a different direction.

"About three blocks away. I'll take his legs, you take his arms."

You walk over, Black giving you more specific directions as you get closer. You reach a shop with a large fish painted on the sign.

"Now we need to get in, to wherever it's smelliest," you say. "Let's put him down here for the moment. You watch him and I'll try to figure that out."

"Be fast," Black says. It sounds a bit like a threat.

You start by walking around. Maybe there's an open window somewhere with the odor wafting out.

Or... the back door could be open just a crack. And a worrying thought strikes you. What if this is the same place Mortimer ran off from, and what if whoever he ran from is still here? You did catch a glimpse of the guildhouse from the street...

And then you hear voices. Quiet ones. You inch closer to the door and listen in. It doesn't take you long to realize what they're talking about.