What do you call a person who works outside his profession?

Technically speaking, profession means the category of work you perform for pay. So, if you're working, then that becomesone of your professions, even if it's outside of your educational history, primary experience, or core competency.

Someone who works occasionally, on the side, or without a long-term contract is usually called a freelancer, consultant, or moonlighter, or hobbyist.