Is there something that he is really excited about or attached to?
My three year old comes and goes with potty training. He is candy crazy. I never let him have sweets (or rarely I should say). He gets a huge sugar rush and becomes very excited and uncontrollable. He knows he gets sugar very rarely. Thus, I promised him a sucker anytime he goes poo on the potty. My girlfriend would give her son three M&M's everytime her son went to the potty. She started giving M&M's when he would sit down. She would reward additional M&M's anytime he pee'd and poo'd. As he became more consistent, he would only get candy if he did something on the potty. Then he became very successful and sitting and pee'ing, and poo'ing was harder to train. Then she only rewarded candy for poo on the potty.
My four year old has a cognitive and speech delay, so I understand part of what you are explaining with the difficulty in getting your point across and keeping his attention with you and others. My 4 year old has been partly trained and then reverts. The last time really helped. I've tried candy, ice cream, movies, take out food, toys, cake, etc. What worked this last time is letting him wear big boy underwear. He got to pick out his favorite characters. I would only let him where Power Rangers, Transformers or Baseball undies when he did what he was supposed to on the potty.