It is heart breaking to watch a pet grieve for another one, especially when you are grieving yourself. I know how you feel exactly as I watched 3 of my dogs search for a dog of ours that had died. They also were depressed and subdued for days before they snapped out of it.
You can help your dog and yourself by going places new like a park or pet store where your dog can be stimulated. Spend extra play time with your dog as their play buddy and companion is gone and not only does your dog not know what's going on, but your dog senses your feelings as well. I know you are doing some of these things already which should help.
Your dog will adjust with your help; it will just take a little while, like it will for you. A new dog might help if you are ready. I'd suggest a younger dog or a puppy so that Chance can be enganged. An older dog might just ignore Chance. If you have a friend with a dog, perhaps having them bring the dog over for a play date might lift his spirits.
I would get some nutrical (nutritional supplement) from your vet or pet store and give him some if he isn't eating normally to ensure he gets the necessary nutrients. In a multiple dog household, dogs learn to eat when it is put down to ensure the other dog doesn't eat it. With only one dog, they will often let it sit longer. If he isn't eating at all, I'd get him a checkup to ensure there isn't a medical issue with him as well. A new pup would likely get him eating normally again pretty quickly.
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