Hello, and welcome.
I'm sorry you're experiencing this problem with your cat.
I have just a few questions to get to know the situation better.
How old is your cat, now?
Does he use the box for urination without any problem?
Are the litterboxes covered or open?
Are the boxes in different locations or near each other?
What litter are you using now?
What do you use to clean the areas he soils?
How does he get along with your other cat?
Are his stools normal--not mushy or extremely hard?
Is he eating, drinking and playing normally, except for this problem?
Thanks for all your additional information and after your reply, please allow me some time to compose and send your detailed answer.