Neutered males are capable of spraying. Even females can do it too. If you have 2 cats and one is not neutered, it very well could be him too. Until you catch the offender in the act, who knows?
If one is doing it, that can spur the other one to start doing as well. So you could have both of them doing it.
There are several things you can do to deter this behavior. Try some of these tips to stop the behavior.
Get the younger neutered ASAP.
You should add a second or third litter box. Many times cats do not like to urinate in the same box they defecate in. Nor do cats like to share litter boxes either sometimes. It always a good rule of thumb to follow this formula (n+1= number of litter boxes) N= number of cats in house.
You can use a litter called cat attract. Attract Litter is designed to make it easier to get cats of all ages trained or re-trained to use the litter box. You can get this a pet smart, petco, and larger pet supply stores.
Very important to clean the areas that has been marked. You need to use a enzyme odor eliminating cleaner such as nature's miracle. Regular cleaner such as Lysol will not work as good. He keeps going to the same spots because he smells his scent there. You must get rid of the scent.
You can also use a product called feliway. This is a pheromone spray that helps cats with aggression, territorial issues, and anxiety. You can spray it on surfaces he has previously urinated on. Pet smart also sells it in a form called comfort zone. This is an electric plug in that puts it in the air.