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?
Take your dog to the vet! There could be serious issues, which could lead to death.
Are you willing to take that chance?