Dear Amy,
Your son's behavior is a direct display of his anger. It's far better to use an adult-size chair as a time out space for young children. Later, at the age of 9 or 10, their room can be used productively as their place for time out. So, I'd advise using a chair instead of his room as the most direct way to address the problem.
Would you consider it appropriate to expect the child to change himself and clean up the chair if he then wets in it?
Dear Pam D,
Yes, absolutley. That would be a perfectly reasonable response.