Well, I'm no lawyer, but the real proof, and it's going to be based on preponderance of the evidence is if the washer caused the sewer backup allowed or not. If you can prove he had prior knowledge of sewage problems, that cannot be attributed to a special circumstance, and he did nothing to prevent recurrence... I think you have a point.
I would also be curious as to how he determined the $2000 if it goes there.
For a countersuit, I would agree on the lost wages and security deposit. I would also push for any other expenses involved in moving and looking for a new place (including mileage and application fees). Also, was anything destroyed or damaged by the backup... and replacement/cleaning/restoration costs are fair game.
