We recently got an adult cat (2-4 yrs) from local shelter & sometimes he poops on the floor near litterbox...?

We recently adopted Arwen %26 he's been FANTASTIC--great with our baby, affectionate, doesn't really scratch anything he's not supposed to, but from Day 1 he's been pooping on the floor near his litterbox...We chalked it up to new surroundings, etc. %26 waited a couple days, then took him to our vet who couldn't find anything wrong w/ him...We've tried every suggestion: cleaning it twice a day, new low-dust clumping litter (like they used at the shelter), new box, 2 separate litter pans, removing the cover, putting it in a quiet/private area, carpet-cleaning the area where the poop was %26 removing the poop from the floor %26 putting it in his box %26 then showing him...He has no problems peeing in both pans, has occasionally pooped in both pans, but for the most part, he poops within 20 ft of his boxes (usually within a foot)...The vet %26 shelter staff have no more suggestions, my husband wants to take him back as he's worried for the baby once he starts to crawl %26 we live in a new house...HELP!
Answers:
they have sprays that you can spray that tells a cat not to do that.

But the best way is to watch him closely and pick him up when he begins to defecate, say NO loudly and put him in the litterbox. Also, try spraying him with a water bottle when he begins to poop outside the box.
This will be a hard behavior to correct, But CAN BE DONE.
DONT GIVE HIM BACK!!!

Cats do respond to discipline!

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Try moving the litter boxes to the areas where he likes to poop, because it is the cat who decides where to go. If that works and he stops pooping on the floor, in a couple of months move the litter box to where you want it. Also, cats are very private by nature. Put the hood back onto the box and when he is ready, put the box in a closet or basement in the corner.

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Well, for one, put the poo in the litter box, so that he knows where to go. Check if there's enough sand in the box. And yea, do what the first person said: put the cat in the box when he/she is trying to defecate.