HELP!! Toddler won't stop playing in poop!!!!

My son is three years old and ever since I was pergant with his sister who is now five months old he keeps playing in his own poop. He refuses to poop on the potty and goes in his pull ups and then he plays in it and puts it everywhere. Some weeks and days are good and he won't do it at all but then we have the bad days where he does it more then twice a day. some of my friends have said their kids used to do it but grew out of it quickly. Not my son thou. I have asked his doctor and she said try giving him more attention and discipline when he does it. Which I already have been giving timeouts for it and been making effort to spend more time with him but he still does it. Just today after I picked him up from school he did it again and put poop all over our nice new sofa. Does anyone else kids do this? How do I stop him from doing this? or is my kid just odd? Help please need advice!!!!!!!!!!!!

Watch him for signs he needs to poop. My toddler isn't even two yet but gives off distinct signs that he has to poop and I take him to the toilet.