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Has a urinalysis been performed? He sounds like he may have a urinary tract infection and if he is not obstructed or has a stone, this is the net thing to rule out. Prostatitis can cause trouble urinating as well. Is he neutered?
Have your vet do a rectal exam and make sure his prostate is not big. If he does have a urinary tract infection, he should be on an antibiotic the penetrates the prostate as this is a place for bacteria to hide and cause re-infection. He should also have a urine culture performed to confirm infection and make sure the antibiotic is the right one for the bacteria if he does have an infection.
Best of luck to you!