It sounds to me like he's Stressed Out. I recognize this because I did the same thing I was stressing about it and in turn he was stressing about it. The more I relaxed the more he did. Stressed is not a good state of mind to learn a new skill. I would take some time off, and then start again. Here is something to consider learning to use the potty is like learning to play the piano you wouldn't scold a child or take things away if he played a wrong note when learning to play twinkle little star, you just keep showing him were the right note is until he gets it. It takes time and practice. I know as a mom you just want to get on to the next stage were he is going in the toilet all the time. Take a deep breath and put yourself in his shoes. When the benefit of doing it outweighs the benefit of not doing it he will do it more and more. It has been quite the learning experience for me as I have been working with my son on this for over a year he is 4 now and finally he is doing it, he still has an occasional accident, I talk to him about how he is going to make it next time. Ask him what he can do to make it ultimately it is his responsibility and he will do it when he's ready. There are several things I have learned along the way. First one was I was being a push over I had to toughen up, in following through with what I said and with having him practice.
One thing you can do is give him a choice in the morning do you want to wear a diaper or big boy underwear, most likely he will choose the diaper or pull up let him know that either way Mommy is done cleaning up his poop. If you make a mess you are responsible for cleaning it up. So then when he goes poop in underwear or diaper he will be the one to clean himself up. You can do this different ways, at first I gave him a bag and wipes in the bathroom and showed him how to do it. I did not do it for him but I did help. The next time I helped a little too, the third time he was on his own to do it. I asked him if he wanted me to stay in the bathroom with him or give him some privacy to do it. Sometimes he wanted me there and sometimes he didn't. If I did stay it was under the condition that he actually did it and didn't just sit and wine and cry and not do it. I learned through trial and error and time. I would cave sometimes and clean it for him, that didn't do either of us any favors I realize now. He is not going to do a perfect clean up job, you just have to let that go for a while. Give him a bath that night. If he is anything like my Son he will fight this big time, he hated cleaning up poop, it was a great motivator. I would talk to him about it and say man that was not fun ha, it would be a lot funner to just go poop in the toilet. Leave it at that.
I learned somewhere to always end the conversation on what you want rather than what you don't want because the last thing they hear will stick in there brain longer. I have liked what I have learned form doctor Phil's show so that's another resource.
So I would do the choice between diaper and underwear for a couple of days, then I would tell him one night, tomorrow you get to wear big boy underwear all day and you will practice so you can get better and better at it. If you want to and you think it might help him you can keep giving him a the choice. I wish I could have all the money back that I spent on pull-ups, they really are just a glorified diaper. I had to be willing to do the work with the underwear. I have also thrown out a good number of underwear because I had my times when it was not worth it to clean them. Even with the underwear, if he poops in it he cleans it. I showed him how to dump the poop in the toilet then put the underwear in a bucket with some cleaner rub it together to get the poop off I do help some with this process, put it in the washing machine the whole bit he will get the idea that it's a lot easier to just do it in the toilet. Then of Course use lot's of praise when he does use the toilet, and tell him that its so fun to keep your underwear clean and dry by going in the toilet then you have more time to play. I am proud of you, are you proud of you too.
Another thing that worked really well was Timing he gets to practice first thing in the morning, then I would check with him the next hour if he said he didn't need to go I would let another hour roll around at the second hour he had to practice even if he didn't think he needed to go. I would give him treats when he went, and my idea of a treat is not candy, but other things that he likes like one mini ritz peanut butter cracker. I do agree with the idea though that we need to get away from using food as a reward, really praise and hugs are enough reward, he will get to were he is proud of himself for doing it and that is the biggest reward. Then when he misses he feels bad about it because he let himself down then I tell him it's OK you are learning you'll make it next time. I also used rewards like he loves superman so if he went in the potty 3 times in a row he got one pair of superman underwear, then as he got better he would get that reward for going all day in the potty. You could give a bigger reward too after he has done really well for awhile my neighbor gave me this idea the potty fairy leaves him a special toy with a note saying how proud the fairy is of you for doing so well. When he has accidents and you see that he is trying still praise the effort. This is my experience I hope it helps you. Be patient we all learn knew skills at our own rate, keeping a positive attitude will be worth Gold over time, and teach him that he can be positive too even when he misses. You might need to go into your room and scream into a pillow sometimes. You can be firm without being negetve. Another idea is when he has an accident say firmly you do that in the toilet take him by the hand and lead him to the toilet he has to sit there for a few minutes, while he is sitting there be lighter again and say next time you have to go pee or poop were are you going to go? My husband made up a fun song that he sings with the kids, it goes "pee and poop go in the potty never never never on the floor pee and poop go in the potty. Then they fill in the blank pee and poop go in the ____ they say it and it's lots of fun, you can do a verse with never in your underwear or pull up. never in your ______.
One last thing at night he still wears a diaper he got good at going potty and didn't want to wear a diaper to bed I told him if he wakes up with a dry diaper then he can try that the next night. Sometimes he wears his underwear over the diaper as a compromise.
There could be something medical going on too.
Good Luck to you I hope this helps