He needs to see a vet. It is worrying that a 6 year old is that lethargic. It is not normal. Also, while he won't get dehydrated if he's drinking and keeping it down, (providing he doesn't also have diarrhea?) 5 days without any food is not good.
Something is going on with him that only a vet can help you with I think.
I totally agree with Ginger. A six year old dog that isn't interested in eating needs to be at the vet ASAP. Only a vet can tell what is going on, they can palpate the abdomen and take an x-ray to see if there is anything structural going on or if there is a blockage.
How long has it been since he has eaten? Is he still going to the toilet? If not, how long has it been since he has?
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