Just watch that when he is going to poop at that time take him out side or where ever u want him to do. Like my cherry do after every meal so within 5 min after feed i take her out side. Ofter some time they will habituate. It wil take time be patient

As soon as he finishes his meal take him outside(terrace or garden) or wherever you want him to poop make him run or walk a little bit he will do within 5 min But be patient it will take time to be a habbit as you are going to start this training a bit late

Don't allow him on couch