Tee I told you about my DD being 4.5 before she would poo anywhere other than her nappy. I believe it was a fear born of being told off by her CM for doing a poo in her nappy when I was just introducing the potty 2 years earlier. So I really do sympathise with you.
This is what I did (oh god the memories!)
DD helped me do a poo chart, this entertained her endlessly as we had crayoned poo's on the chart and it was stuck next to the toilet. I didnt push the chart, I just said that when she did a poo she could have a sticker and if she got 2 stickers she could have a treat.
Then, she would need a poo so I would encourage her to sit on the toilet. I told her that I would read her a story, so I would sit on the floor and read a story. This was beyond tedious and laborious but it got her relaxed and sat on the loo!
Then I had the ingenious idea of a puppet show with her favourite soft toys.
you now think I am insane...
I would hide one side of the door and the toys would tell her a story. Sometimes this went on for 15+ minutes!
Basically it changed the toilet environment from being scary to being a place she could relax.
It took a while, probably 2 months from start to finish (but she had major issues to start with and I was not going to pressure her) and my god I hated those bloody story/puppet sessions, but ultimately it worked.
Can you think of ways to make it fun so he can relax? Would he respond to drawing a big poo on an A3 sheet of neon pink paper?
Does he like pot-cat ornaments from the pound shop? (DD's first reward, her request) 
I agree with what was said the other day on your thread - back off at the moment for both your sakes, remove the issue altogether and then start afresh.
if after reading my confession anybody wants to take the piss, please do, but be gentle with me 