Hi Betty,

The most common reasons for a dog to start defecating in the house after being previously housetrained is a parasite problem such as worms, coccidia or giardia. You can read about these below:
http://www.thepetcenter.com/exa/worms.html

http://www.peteducation.com/article.cfm?cls=2&cat=1622&articleid=727

http://www.missionmedvet.com/encyclopedia/giardia_canine.htm

I would have this checked first before deciding it is a behavioral problem.

The new behavior could be your dog's way of getting more attention as a dog doesn't care if it is good attention or bad as long as it is attention. Be sure to clean up the scent completely with a pet odor eliminating cleaner and to practice praise when your dog eliminates where it is appropriate. Remember that disciplining your dog if you do not catch them in the act is confusing for your dog and should not be done.

If a medical condition is ruled out, you will have to retrain your dog all over again, though cleaning up the old scent and paying more attention to your dog may make a world of difference.

Your boy is also becoming elderly and as such could start experiencing Cognitive dysfunction syndrome. Here is a wonderful site that explains it well. Please see this site for more information.
http://www.swiftwaterfarms.com/swiftwater/p22CanineCognitiveDysfunction.htm

Check the symptoms of CDS as your dog may have this which is contributing to his problem as well.

Hope this helps.

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Thanks,
Jane