My dog had surgery for that on April 1 and my vet said there was a possibility of that happening, but that it would be temporary.

Thankfully it didn't happen to my boy. Maybe talk to your vet and see what he/she says.

If he was catheterised, he may be blocked again and it's overflow caused by a full bladder against more blockage. Can you tell if his bladder is full and distended?

you probably have your question answered by now, but often feline bladders are distended so long during a blockage that they lose the ability to contract for a while after being emptied, and consciously voiding urine will be difficult. Often, for up to a week or more, their bladders will simply fill up to the point of being totally full again and (if all goes well) will empty gradually once they are full simply because there is no more room- resulting in an almost constant dribble of urine. Once the bladder recovers its muscle tone it will be able to empty itself more efficiently and the dribbling will usually stop.

.I don't think I've thanked you before lzrddr for your contributions and comments but we're really fortunate to have a veterinarian take the time to offer
their opinions.