My father in law (75) is unable to express passing urine. What is the reason?

Could be a prostate issue.

If he is unable to pass urine could have blockage from buildup.They may need to snake his track to open.Sounds harsh but it happens.When he does Urine keep a eye on the color and amount.The darker it is the worse it is.Maybe kedneys shutting down.He needs to see a doctor now.

If your father has not urinated in 4 hours, call your doctor right now. Probably something that will pass, but be safe.

A frequency in urinating is not unusual for a diabetic, however I do have to agree he may have a UTI or possible stones. Either way you need to take him to his Dr. asap. I would take him to which ever Dr. handles his sugar. Good Luck

If this just started happening, take him to be checked for a UTI- "Urinary Tract Infection" immediately, today, do not delay.