Why did they tell you he was unable to void? Does he have an infection? My wife still voids a few ounces (2 to 4) about once a day. When she stops voiding or if it burns when she voids, we flush her bladder with an antibiotic (gentamicin). Talk to a urologist.
The reason I am asking is, most ER doctors (may not be fair to say most, but that has been my experience) don't think about the fact that oral or IV antibiotics aren't going to get rid of a urinary tract infection if you aren't producing urine. The antibiotic has to enter the bladder through the urine to be effective. No urine, no antibiotic inside the bladder.