what the poop is going on

I have a 10 year old nutered male cat and and he was great with the litter box until about 6 years ago he started pooping outside the box now at first we thought he was pissed at us for bring another cat in the house but he didnt start doing this until at least a year after we brought in the other cat who is a spayed female and they always and still so play and get along now my male cat doing the bad deed is 20 pounds and large for a cat vet has checked him out completely and says hes fine and hes very social with us doesnt seem un happy and we have 4 litter boxs in the house in different areas all privet and out of human sight and clean now ive tolerated this for 6 years because iys just poop and just ouside the box never any where else until just a couple weeks a go whenn he peed just out side the box and is now pooping farther away from the box i love this guy but if we dont find whats wroonog hes going to be put down iam at the end of my rope anyone who has any idea what this could be help me and pete before its to late

Oh dear.
Please please don't put him to sleep because of that.
That's just ridiculous to even think about. I totally know how it is. In fact..I've got almost the same problem in This Thread.
There are a few ideas being given to me in that thread. Check it out.
Hope it helps some.
But again, and I cant possibly emphasize this enough, PLEASE do not put him to sleep because of it.

thanks i dont want to iam just 6 years of fustration builds but every time he comes for love or to talk i alway say to my self what were you thinking. but my husband was a farm boy and he does love cats but doent have the same compassion we do. Ill keep trying i promise

Did you check into any of the ideas given in the thread link I gave you?
They are great ideas, and I've already put a couple into place to help my cats pooping outside the box problem. Hopefully they work.
I know it is frustrating. Just don't let it get to you that bad.
Sheesh...I'd never think of putting him to sleep because of it though.
I had even thought about making my entire closet a 5X5 container full of litter to use as a litter box. Something like a sandbox you know?
But that seemed a little impractical.

Just keep trying to help out the situation. It'll all work out somehow.