I would refrain from leaving him outside unsupervised. Theft is a worry (especially with cute, wiggly puppies) as is worry of predators, bees (many boxers have allergic reactions to bee stings where their entire heads swell up), poisoning by people who mean ill toward animals, etc.
You are correct in your thougths that 8 weeks old is too young to hold it for an 8 hour work day. Usually young puppies need to be taken out once every 4-5 hours.
Can you have a neighbor come over and let him out?
What about hiring a dog walker or a pet sitter to come over and let him out?
Taking him to Doggie Daycare?
If these are not options for you, I would purchase an exercise pen and line one side of the floor with newspapers so that he can potty on them and not have to lie in it. This is obviously the least desirable option, because it may make potty training harder later on, but still much better than inadvertently conditioning him to lie in his own potty.
Good luck, hope this helps!