This is definitely workable, though I think that not allowing him to relieve himself for 9 hours straight on a daily basis is a bad idea, and one that might lead to a UTI (urinary tract infection).
Do you work far from home? Can you not take a break in the middle of the day to go for a quick run and let him potty? As was mentioned above, maybe get your Dad (or a neighbor, friend, anyone) to come over in the middle of the day and let him out?
I'm no doctor, and am far from an expert on boxers, but I see a four/five hour of "holding it" as no problem, and eight as about the limit on what you'd want to do on a semi regular basis. I know that on days when, for whatever reason, I don't get home in the middle of the day to let my dog out, I'm stressed/worried about my buddy on the way home.
Now, the other part of this is if the dog is crate trained, and I kinda suspect he's not (just based on him being a "spoiled" 5 year old guy and you're not sure how long he can hold it, sounds like he's had a pretty good life : ) ). I think if you're planning on leaving the dog out in the house for 9 hours a day, you'd be wise to invest in a steam carpet cleaner or something like that. If there's any option availiable to him, I don't think the dog will hold it for that amount of time, and I think you'd quickly discover what location in the house most approximates "outside" for him. But if he's in the crate, there's a powerful incentive for him to not potty. And honestly, if I was leaving the dog alone for that amount of time I'd want to crate him anyways, just out of fear that he'll get bored, get into something he shouldn't get into, and then you've got a sick (or worse) dog on your hands. If the dog isn't crate trained, than thats' a whole nother issue and you should probably start working on that with him.