Dogs (and us) learn by making mistakes. As hard as it may sound; try and not show any attention to the possums (walk up to him, then just walk away) and he'll soon learn that it doesn't interest you - and very soon it wont interest him either if it's not making you happy.
You'll loose more deer than you'd normally get without the dog while training him, that's just how it is. But at some point he'll turn that tide and then you'll wonder how you ever hunted without him.
Good luck
kj
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