Why Does My Cat Do This?

I hae 6 cats, all inside (3 litter boxes changed frequently), all fixed. The 8 yr. old male uses the box during the day, but at nite, he poops and pees in my kitchen, ea. in a different spot, but always the same spot for each activity. He has also begun peeing on my living room carpet in the same place each time. Reprimands don't work. Nothing has. He appears to be healthy. We think he may be a little slow, he kind of acts as if he is a little retarded compared to the other cats. He is very sweet, loving and docile. His behaviour has me upset, my house foul smelling and I have no idea how to get him to stop. Has anyone been through this and if so, was there a resolution short of getting rid of the kitty? I have had him since birth. There are no new felines added to the family recently, so it is not that he feels threatened. I am at my wit's end. CAN ANYONE HELP ME PLEASE??? THANKS SO MUCH!