I have 2 boys, and didn't rush either of them but let them choose when. My oldest would continue to put himself into a pullup before his BM until he was 4. He knew how but just wasn't ready to poop in the potty. Then, on his 4th birthday, he agreed. This is a little later than most people would like their child to be trained, but from what I understand you can start as early as you want but they will not be trained until they are ready anyway. Also we raarely had accidents to clean up. My youngest son was basically the same. They both peed in the potty early enough, around 3, but didn't want to sit to have a BM. I let them have their pullups until they were ready. Bottom line, they were finished training at 4 yrs, they were never stressed about it, and neither had any real accidents (all total for 2 boys, maybe 3 or 4 accidents from waiting too long).
Bottom line, if you can wait, they will train themselves when they are ready and it is a lot less stressful for everybody.
Tell him to let you know when he has to have a bowel movement so that you can put a diaper on him. You do not want him to start holding his bowel movement. That results in a health issue. I would put his diaper on, take him to the bathroom and sit him on the potty with his diaper. Leave and give him his privacy until he's finished and then clean him up. He will one day go on the potty. You will not have a 4 year old who poops in a diaper. Kids get these little funny ideas in their heads. He's doing great!
If you were driving your car and headed down the wrong road, you'd make a mid-course correction and return so that you could go down the right road. Parenting is no different. Some people think that, once they've set a course re: some parenting decision, even if it proves not to be helpful, they're obligated to stick to the decision. You should change course. You were a bit ahead of the game with your son. It was a good try, but he's just not ready yet. There is absolutely no substitute when it comes to toilet training to wait until the child is ready, and this is not usually an overnight sort of proposition. It's a process, and many youngsters have not fully mastered toilet training by your son's age. It's OK to return to the diapers, for all or part of the time - it's up to you. I see that you realize it's imprtant to maintain your equanimity, and using self-talk (e.g., 'stay calm', 'relax') can be very helpful. In fact, learning the skill of maintaining equanimity in the face of our children's behavior is a skill that will serve you well throughout their chikdhood and adolescent years.
The last poster was right, you don't want it to get to the point where he holds it in. However, just for a day or two try this: When he poops in his pants, tell him that he is suppose to go in the toilet like when he goes pee. Next, hand him a box of wipes and clean pair of underware and tell him to go into the bathroom and clean up his own mess. After he tries his best to clean up, tell him he wouldn't have to do that if he would go BM in the toilet. After a couple of times, he might just change his mind about pooping in the toilet.