How to handle a dog quit hunting?

Sounds like a tough one Skrag. Once you have ruled out any health issues, you might consider evaluating his feed. He might need a higher octane feed at this point in his life. I had a dog act similar one time and figured out that he wasn't drinking water because we were putting Clorox tabs in it to keep it clean. He was chronically dehydrated. Stopped the Clorox tabs and he got better.
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I have seen solid dogs that decided they were not gonna waste their time huntin with silly acting pups.
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You might also consider leaving him at home a few times and hunting without him. Come home and make sure he smells hog on you and knows you went huntin w/out him. You could haul him and turn the others loose and make him watch. Maybe it would relight his fire.
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It is my opinion/observation that certain dogs can be very intelligent/complex creatures and require us to out think them. Intelligence is a common trait in the good ones. It sounds like you have a good one.
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Just a few ideas to consider. Hope you get him figured out.