Gosh, I know that behavior is very frustrating !!! How long ago did he get neutered? At only nine months old, he can still have an accident now and then. If he were neutered very recently, he might be out of his routine for a day or so after coming home from the vet, but most likely it is because he is still young and just made a mistake.
Make sure the carpets are cleaned of all sights and smells of his lapse in memory and just go back to the same method you used to housebreak him successfully before. He should be back to his usuall routine soon.
