Well, you got lots of advice here! I will share what worked with my kids.
1) Stay relaxed. I am a very relaxed parent, and I always say, this too, shall pass. And it always does. I don't get parents that try to push their kids.
2) DS finally started pooping in the potty when we shut the door. He was just really uncomfortable with us standing there looking at him. How funny that I didn't get that, and how I can totally see know how he just couldn't poo under the pressure. I set him in there with a book and he yelled when he was done. Perhaps the pressure of having someone there waiting for him to poo or pee is intimidating to him??
3) DD we just made her clean herself up. She would cry, but she couldn't leave the bathroom until she was clean. (and the bathroom was clean, I would go in there and bleach later). It took about 3 times, and she was done. I just told her, now you are 3, and 3 year olds use the potty and put her in underwear. She peed, I made her clean it up. She hated that, and was swiftly trained.
Whatever you do, good luck! And, honestly, I think if you just lay low and let him be, he will eventually do it on his own.