Let him try to impoverish his towns and take all their able-bodied workers to be killed in war. Two or three towns along he and his slow-moving treasure caravan and small mob of unarmed peasants will run into the collected forces of the armed rebels who object to this treatment, which will be numerous enough that he can't hope to fight them. Tyrants don't rule (or live) for long unless they're smart tyrants.

Maybe warn him that it would happen that way first.

Let him try to impoverish his towns and take all their able-bodied workers to be killed in war. Two or three towns along he and his slow-moving treasure caravan and small mob of unarmed peasants will run into the collected forces of the armed rebels who object to this treatment, which will be numerous enough that he can't hope to fight them.

Maybe warn him that it would happen that way first.

Tyrants don't rule (or live) for long unless they're smart tyrants. Ruling an evil kingdom takes guile, an iron fist, and the ability to keep absolutely every underling from putting a dagger in your back (because it only takes one).

The vassals under the king are also powers in their own right, and they have their own agendas that aren't going to be compatible with a king who just takes what he wants. Most of the towns and citizens of the kingdom are actually not directly subjects of the king – they're going to be the subjects of the dukes and counts under the king, and seizing their gold and able workers is going to be opposed by the noble to whom that gold and those labourers actually belong. Unless this new king wants to (try to) strip his vassals of their titles and seize their lands so he doesn't need their cooperation to rule, he actually has very little direct control over the kingdom. And stripping vassals of titles tends to just make them angry and raise an army to depose you.