I would suggest having a company come in and clean your carpets professionally- make sure to pay for the extra de-odorizing service.
Then get a blacklight. Every night use the blacklight to see if there is any new stains in the apartment and use a good cleaner like Nature's Miracle to get them up.
If the whole place smells like pee it will be very difficult to get him trained.
__________________ Armani & Chloe |