My son is almost three and I tried in the past to potty train him; he just wasn't ready.
He's only recently started showing interest in the potty again and goes successfully. I don't push the issue though as I figure he'll go when he's ready.
My daughter however had a fear of pooping.
She was potty trained for pee at 2.5 years old and wore underwear all the time. However she would ask for a diaper to poop in.
I refused to give her one when she was about 3.5 (yes, for a year I changed poopie diapers with her figuring she would eventually go on her own) and for 5 days she didn't poop and was in pain from holding it.
I stuck some glycerine up her bum one day and told her it would help her poop on the potty. She went 45 minutes later and never pooped in a diaper again.

I think because she was always holding it for so long before going on the potty, it hurt coming out and she associated the pain with the toilet. The glycerine must have helped lubricate it and she went pain-free.

Every kid is different. Just give it time and don't stress. He'll know when he's ready.