My dog is dying and 2 vets cannot diagnose after blood/urine/xray - PLEASE HELP!

I have been to two vets and neither can determine what the issue is (besides doing exploratory surgery), details below, PLEASE HELP save the life of this wonderful companion.

Lyon, Flat Coat Retriever, 9 years 11 months old, male, intact.

About three weeks ago he began excessively drinking and would get up 4-6 times at night to pee. Shortly after he stopped eating his dry food, the same one he loved just a week prior. He lost interest in all food, but would still beg for his morning Greenies dental chew and the occasional human food. He went from 74 lbs to 70 lbs in the first week, but has been stable at 70 lbs for two weeks. He now rejects his daily vitamin, fish oil, and glucosamine/MSM. To get him to eat, we have been feeding him boiled chicken or salmon with rice, but he would only finish half portion.

He still looks forward to walks, but moves at a sluggish pace and would stop to rest. He would vomit white foam (happens about 2 nights a week), and is generally tired. In the past week, he appears to be constipating. He has a round lump of mass about 2x1 inches around the left bulbourethal gland that does not go away - cytology report found some histiocyte, round cells, but no obvious indicator of cancer - and ultrasound found it to be a mass inside the abdomen cavity and not a gland. He has been on antibiotics (sulfamethoxazole) for a week.

The vet ruled out Cushings, Addersons, diabetes, and said his prostate is slightly enlarged due to age but not abnormal. Please, if you have any advice on what he could be suffering from, or what to do next. We feel surgery is too premature until we have a better idea of what we are dealing with. THANK YOU.

Values from his blood work that are abnormal:
albumin 2.3 (range 2.7-4.4)
A/G ration 0.7 (range 0.8-2.0)
Alk phos 195 (range 5-131)
magnesium 1.1 (range 1.5-2.5)
precisionPSL 159 (range 25-140)
WBC 22.5 (range 4.0-15.5)
platelet count 524 (range 170-400)
neutrophils 19800 (range 2060-10600)
monocytes 900 (range 0-840)

Urinalysis:
yellow, clear, pH 6.0 (range 5.5-7.0), but protein is high at 1+.
Negative on crystals, ketones, WBC, etc.

It could be this? Has the vet tested for this? Prostatic Adenocarcinoma in Dogs

I'm very sorry to hear this .

I think you will have to consider abdominal surgery soon to see what the mass is , could he have swallowed something that is stuck , been injured in anyway ? The blood tests show high platelets and High neurotrophils which the mass could be infected .

It could peritonitis . You really need to speak to your vet again .

I hope that Lyon will be fine, what a beautiful boy he is.