Hi twomutts, as far as the prostate goes, most hormone related problems don't happen in sterilized dogs, but that's not to say it CAN'T happen. He is an older dog, so prostate problems aren't totally unusual.
Metronidazole is a great drug to treat stomach problems, so I would stick with it for now.
But I understand what you are saying, and I believe you are on the right track with thinking an enlarged prostate could possibly block the bowel and cause him to have to strain to go poo. I am not 100% familiar with where the prostate is located on a male dog, but your idea makes total sense to me.
I would do some research into seeing if there are medications to reduce the size of a dogs prostate, as I know they make them for humans (thanks to lovely prescription commercials!
). In all honesty you probably are going to have to do further testing anyways, probably an ultrasound, to see if there is a tumor or some other form of prostate cancer. I'm sure that's not what you would want to hear, but considering his age it is something you need to check for.