They want the Option C Print Out anyways, to prove that he isnt on welfare, so I assumme that you are asking what he should send instead of a letter of employment.
If he is reporting all of his income properly for tax purposes, then a Notice of assessment or two will show them what he makes annually. If he is incorporated there are a lot of things that he could send like financials, copies of the income statement, copies of the incorporation documents showing that he is a shareholder in the company. If he is not incorporated and only a sole proprieter then he should write a letter explaining what it is that he does, how long he's been doing it for, etc just some details of the business. Then if Etsy somehow summarizes the income for him he could print out statements from them showing his etsy income for the last couple of years. In regards to farmers markets and things like that where he earns mostly a cash income he should have records of how much he has made from those booths. He should have records of Income vs Expenses and things like that that he can send to prove that he is earning sufficient money to support the both of you. If he has his own business cards, website or anything you can include any information like that along with the letter.