My guess is some kind of worm. Common in dogs is the tapeworm which they get by eating an infected flea. They eat whatever the dog eats and since this happens, the lack of calories is making him tired and sluggish. It is easy to treat but needs to be treated. I would look for irritation around the anus. You can look up other symptoms online. Best wishes at the vet tomorrow.
Sent from Petguide.com Free App