There are many reasons that a dog will stop eating well. Most of the time it is not that he stopped liking the food even though new food might get him eating for a short time. Spoiled food can affect appetite, but usually, poor appetite has a medical cause.
Possible causes include gum and tooth disease, mouth lesions (ulcers, tumors), nausea caused by stomach ulcers (does he get any medications including for pain), systemic infection, kidney and/or liver disease, bladder infections (cystitis), pain from any cause, tumors/cancer, gastrointestinal disease, emotional stress (boarding, owner away/sick, other companion animals sick or away). Some of these commonly have additional signs such as diarrhea and/or vomiting.
The above list is not all inclusive, but does indicate the wide range of problems that can cause anorexia. Not eating is a sign of a problem rather than the actual problem. Diagnosis of the specific cause is important. If a dog fails to eat for more than a couple days, a veterinary exam is in order. This includes a history of recent events in the dog's life, a physical exam and diagnostic testing as indicated. A fecal analysis (take a fresh sample with you), urinalysis, complete blood count and blood chemistry are usually part of an inclusive medical exam.
Let me know if you have follow up questions.
I think it is fine to wait to see your regular vet on Monday so long as Willie is not vomiting a lot or having severe diarrhea. Your regular vet knows you and Willie making him/her a better choice than an emergency vet who know medicine, but not your boy's history.
With a history of drinking more than normal, blood work is indicated. If you are in tick country, Lyme, Anaplasma and Erhlichia tests are also worthwhile. Theses come packaged with a heartworm test (Idexx) that many vets use.
You can entice him with tasty morsels including people food (chicken eggs, etc.) to see if he will eat something. Alternatively, feed his dog biscuit treats or canned food.
If you are able to get a temperature, human thermometer used lubricated (vaseline, water) rectally, normal is 101-102.5 and over 103 is a low grade fever. If it is much higher than 103, consider a vet sooner than Monday. Not eating for a week is not usually associated with a fever and no other signs.
Also, check his gum color. It should be pin, not white, gray or dark red. Push your finger against the gum just behind the upper incisor tooth. It will blanch white under the pressure and should return to the original color (pink) within 2 seconds. Longer than 3 seconds indicates dehydration and/or decreased perfusion (circulation)