This is unusual. Dogs don't typically want to soil their own areas. First, has he been neutered yet? That will help with the urge to mark his territory. Next, clean the flooring thoroughly with an odor removing agent; I'd suggest one specifically for pet odors. Then, you'll need to restrict his water and food intake before and during the time you're away. You might even try checking into one of those self feeders that will release a specific amount of food at a set time and let him eat 10 or 15 minutes before you normally get home.
Someone here (sorry, I wish I could remember who it was so I could give them the proper credit!) also had a wonderful suggestion for this problem. Get angry at the "present." Scold it, yell at it, and make sure your dog sees you doing it. Give him the impression that you intensely dislike the stuff.
You might have to consider confining him to a smaller area, like the bathroom, when you are gone since that decreases his area and should deter him some.
And, perhaps most importantly, praise him like mad when he does his business outside. Make it a big deal so he'll want to hold it and get the positive attention for going where he's supposed to go.