Not to sound rude, but obviously there is something wrong, or you wouldn't be here asking.
Lethargy in dogs is never a good sign. How old is your pup? Age can be a factor, but if this lethargy and licking is sudden, you should definitely get him to his vet. Sudden onset lethargy has a number of causes, too many to list here, and most of them bad.
The causes include (but aren't limited to): canine distemper, parvovirus infection, gastrointestinal parasites (can lead to blood loss or inadequate nutrient absorption), bacterial infections or malnutrition (can cause low blood sugar), and gastrointestinal disease. Also, lack of fluid intake can result in dehydration. Which can happen from poor absorption, not just forgetting to fill the bowl...
About the only good cause is pregnancy...not the case with your little guy.
But yes, I would say going to the vet is your best bet here, and soon.
Hope this helped! Good Luck!