Can you take him out on leash to where the evil groundhog lives? Maybe if he can get to it's location to get some sniffing done, then maybe it won't be such an obsession. You can use tasty treats to reward him when he is done sniffing and heads home. Something about groundhogs drives mine crazy, even worse than bunnies. Or, maybe you can use a have-a-heart trap to catch the hog and relocate him to a new safe location.