.. How do I talk to the prisoner?

Apparently, his breath leaves much to be desired.

But what does he desire?

A breath mint, maybe?

Now, where's the logical place to buy some breath mints.

It's the same place where you can buy a sword and a shovel.

Go to the store in the far part of town. Talk to the storekeeper and ask him to give you a breath mint. In order for breath mints to be a buying option, you must have already tried to talk to the prisoner.