Just list his job on the job history. You do not have to provide any documentation from his employer - just list the job, address, etc. At the interview they may ask about his job. He can tell them what he did for his work; unless they ask specifically he does not have to say he was working 'under the table'. In any case, the visa officers should understand the conditions of the country they are working in and won't be surprised.
As other people have said, be honest. Include the job in his job history; if asked directly, 'was it under the table?', admit it.
Also keep in mind the visa officer who interviews your husband will be more concerned about whether you and he have a genuine relationship. If you don't list his current job, the visa officer will think 'hmmm, he's unemployed. Maybe he won't want to work in Canada either. Or maybe he only wants to come to Canada to make more money.'