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My suggestion would be to get your x to sign a rental agreement and have it back dated to the date he started to "rent" from you. The rental agreement backs up your tax return.
Your mortgage is on your credit report, so yeah, you should have the rental agreement and the copies of your tax returns to show it washes.
Good luck!
From my understanding, if you have filed taxes, the net income/loss will be included since you will have filed taxes on it for a few years. BUt, it depends. It may not be counted in your DTI. However, be very careful in your calculation of the rental revenue and how that translates to income or loss. If you have a loss, I am not sure if they will inquire as to why and how and may want to include it in your DTI.
Ask your broker once they have analyzed your specific situation.