Answer: It's obvious your dog controls your household as the cat controls mine! I've not heard of dogs being so thrilled with citrus. Maybe squeeze some fresh lemon on his kibble...
But seriously, if you can't train him to stay away from the plant, perhaps you can put some barrier around it. Cover the soil with a mulch such as bark chips, spagnum moss, small gravel, etc. Whatever is uninteresting to him. You can still water through it. As for the leaves, this may sound ugly, but can you install some sort of chicken-wire fence around the plant until he tires of it. At the same time, try some commands, as in NO, anytime he goes near!
Also, where did you get the plant? Perhaps it or the soil smells like the nursery's dog or cat? Animals have tremendous powers of scent and maybe it's not the lemon tree, but something on the lemon tree. You could try hosing it off in your shower. You might check with your vet to see if the dog might be missing some nutrient that citrus leaves and dirt satisfies. Finally you could try a homemade garlic or pepper spray, which most animal pests find distasteful on outdoor gardens. However, since the plant is indoors, the smell might be unpleasant for you. And your dog might like it! Sorry I can't be more definate, but good luck!
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