I would first rule out a medical reason and make sure he doesn't have a bladder infection. If that is not the case thank back if there had been anyone or any animal at your house prior to the incident. It sounds like it may be a marking his territory thing more than anything. Any of your friends have other male dogs that may have incited this behavior. Sometimes just the smell of another male dog will do it. In the mean time, here is a site that gives a bunch of suggestions on how to clean the area, all very good methods so you can choose your own instead of me choosing one for you:
http://www.stretcher.com/stories/971009g.cfm
Good Luck!
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Good Luck &
Best Fishes
