I would go with crate training for him. I don't see any other way. If he can 'hold it' throughout the night - he can hold it during the day while you're gone.
I'm gone from 6am until 4pm (10 hours) and my dogs are fine holding it til I get home. And they are not desparate to 'go' after that time either.
It won't be easy but it will work if you persist. Good luck!!