I am very worried about Blue. I recommend that you follow up with your veterinarian for an abdominal ultrasound and prostatic biopsy or aspirate. Chest X-rays may also be helpful.
In non-neutered dogs with enlarged prostate, my top possibilities include benign prostatic enlargement (hormone influenced), prostatic cysts and prostatic infections (predisposed by hormone influence), and cancer.
In neutered dogs with enlarged prostates, my top concern is cancer. Cystic disease and infection are also potentials but I would be concerned about cancer until proven otherwise. While there is some controversy over this (some studies show equal risk and some do not) - it appears that neutered male dogs may
be up to four times more likely to develop prostatic cancer than non-neutered dogs. For all dogs, the risk of prostatic cancer is at 0.29-0.6%. Thus the overall risk is low whether neutered or not. For this reason it is not typically discussed prior to neutering. Also neutering reduces the risk of many other diseases and causes of death and FWIW neutered animals typically live 1-2 years longer than non-neutered pets.
Back to Blue...
The straining to defecate is more likely from colonic inflammation. The inflammation of the colon with make the pet feel that they need to defecate. If there is a prostatic tumor, they often release chemical mediators that lead to generalized inflammation and diarrhea and other signs can occur. Also if there is abdominal lymph node involvement, there may be no way of addressing the diarrhea and intestinal inflammation without addressing the prostatic disease.
The choice of metronidazole is a standard antibiotic therapy for diarrhea. I would pursue further diagnostic understanding of the prostate. The answer for the diarrhea might be there. You could also simultaneously work up the diarrhea with fecal test, giardia ELISA, fecal culture, abdominal ultrasound, etc...
I hope that this helps. Please let me know if there is anything I can do.