I was able to train a neurotic cocker spaniel that i seen maybe for four hours a day twice a week to go outside. She was super submissive. (It didn't help that most of her owners mistreated her.)

She would pee as soon as you went to pet her or if you touched her.

What i did when i'd arrive at the house once i figured this out is i'd ignor her if we were inside. Since she was excited to see me i'd walk with her to the outside and pet her and praise her. She could pee to her heart's content. Then once we were back inside i'd pet her if she wanted me to, but if she started to pee i'd immediately stand up and ignore her. If she still wanted me to pet her we'd go right back outside.

While i was there on a weekly basis this worked. (Well at least with me, but her owners were too stubborn to try it my way and insisted she'd never be housebroke.) Then when i wouldn't be there for months at a time the training went out the window... of course. Her owners didn't use the method and continued to mistreat her.

Remember... praise is the key in training a dog to do just about anything.

You already have mortified your new charge, so i don't know if my usual method would work. ( Taking the dog back and forth between where they are supposed to go and where they aren't a few times and alternating the praise and scolding. ) I think in this case ignoring the bad would be a good idea. At least for now.

Have you made sure that he doesn't have anything like a bladder infection? Soemthing like that would cause him to not be able to hold it. (Not that the fear is a far fetched idea.)

Maybe i missed it, but how many hours are you gone? Is it possible for you to come home during your lunch hour to let the dog out? Do you have someone you would trust with your own life that you could have let the dog out?