Honestly, Just write all the details in the letter; given them all the dates and incidents and the fact that he has now quit his job and stays with you. Provide all the bills under his name that come to the same address as yours. Anything you can think of just submit it to them as proof. And hope for the best at this point.
As for the immigration lawyer, I agree it's a complete waste of money unless the case is complicated; which yours isn't. But in this case maybe go in for a consultation and see if they can offer you any advice. (hopefully they don't charge you for a consultation).
Wish you all the best. Don't give up just yet.