mysterious problem

[twomutts]

Hi, I am new here and really hope I'm posting appropriately. I'm looking for some insight into this problem. My wonderful 12 year old border collie/Australian cattle dog mix, Blue, began vomiting in the early morning, Thursday. (First some solid stuff; food and grass, then mostly clear, frothy mucosy liquid.) He strained to move his bowels but produced only tiny yellowish blobs. He was obviously uncomfortable, panting and restless. He refused food, even the scrambled egg I made him. (He loves them.)
I feared a blockage and made a vet app't.
Friday morning, (yesterday) I took him to the vet who palpated his abdomen thoroughly and said he didn't think it was a blockage. (Blue was "empty") He suspected colitis and wanted to do bloodwork to check liver and kidney function, before medicating. This revealed normal function, with only one type of white blood cell elevation that he suspected was from the stress of the situation. There was no fever. However, he said Blue's prostate was huge. Blue was neutered in his first year, so I am puzzled by this. He prescribed metronizadole (aka flagyl) and a bland diet and told me to call him Monday.
Well, it's now Saturday afternoon and nothing has changed. He is still unable to defecate (although he refuses food, so is still empty) but he strains trying and his back legs tremble from the effort. He is drinking water but occasionally vomits it up. I don't think the meds are improving him and I would think they would have by now, if it was simply colitis. Also, I don't understand why his prostate would be enlarged in the absence of testosterone. He is definitely sick, lethargic and uncomfortable. I am sick with worry. Could a simple enlarged prostate cause these symptoms, or are we talking about cancer here? Would the prostate cause him to keep trying to poop, or is he possibly blocked in the small intestine? I've done some reading but I still feel mystified.
Thanks in advance for your thoughts.