1-make sure he's not constipated. lots of drinks/popcycles. my kids love eating dried plumbs (prunes). Pineapple or apple juice is also good.
2-This is what I did with my boy. It's kinda icky, but it's tons better than cleaning up poop.
Boy sits on potty. Boy pees.
Mom: ! You made bubbles, good job :)
Boy: Look, Mom, bubbles!
Mom: Can you make a poopy too? go like this [grunts]
Boy grunts (with or without pushing out his poop)
Mom: Oh! I think I can see it! Is it coming?
Boy pushes a bit
Mom: I can see it!
Boy pushes it all the way
Mom: Oh! it splashed! hehe. Did your body make any more poopies?
----etc
Then when he's all done, I wipe his butt. I always tell him how much easier it is to wipe poop when you go in the potty...and how hard it is to wipe it off when you go poop in your underwear or in a baby diaper.
Then, Mom: Ok. I'm done - let's say g'bye to your poopie.
Boy flushes.
Boy and Mom (in silly, deep voice): Bye, Bye poopie!!
It really helps him a ton for me to watch for his poop and tell him when I can see it...and when I can't see it anymore.
If that hadn't worked, I would've given him the choice of wearing a diaper or going poop in the potty. No one stays in diapers forever. He'll get it soon.