Is he currently having difficulty ? Has he passed urine with blood in it, shown discomfort upon urination (usually meowing or making other noises while in the box)? Has his urine production decreased or stopped?
If there is a current problem - you have just a matter of hours to get him to a vet and unblocked . It's considered a medical emergency, day/night/weekend/holiday.
If this is a past problem that you're hoping to avoid again - let me know that too.
Many have forgotten this truth, but you must not forget it.
You remain responsible, forever, for what you have tamed.