Just my initial impression of the picture you've presented- You're turning his pooping into a power struggle. You're trying to control his toilet behavior and he is outright rebelling. As parents we need to be very selective in the battles we choose to wage with our children. Often times we get caught up in power struggles that endlessly backfire because we, as parents, can't disengage long enough to trust our children to do the right things. Trust isn't borne overnight- we should know this- and we should give them time to come around to "proper" ways. Children, when treated proper, desire to please us and to gain independence.
Never make using the toilet a battle. It is a fight you will lose. You should check with his pediatrician, of course, to eliminate medical reasons for his behavior. But, I think what is most effective is patience, kindness, and positivity. It will take time to get him on track, provided there is no underlying medical issue, so you must be patient and kind. Negative reinforcement of any kind can cause serious issues with his toilet confidence, and also cause confusion.
It may be possible he passed a large painful stool earlier in his potty training days. My brother had that fear, and it turned into a fear of a monster in the toilet that "hurt his butt"........ Also, if he's had any accidents at school the other children teasing him would simply deepen his resolve to not replicate the incident that got him teased. As the previous poster noted: it could be a control issue too- children have very little control over their lives and control issues tend to manifest themselves in toilet issues and food issues. If it is a control issue the very best thing you can do is back off and give him some space to arrive at the decision to use the toilet on his own. Let him know where the toilet is, where the paper is, and that you are right down the way should he need any assistance while using the toilet. Leave it at that, and let him choose for himself how he wants to deal with his poops.
On a side note- if you use a warm cloth to wipe his butt- he may actually enjoy it. Start using a cold cloth, or wiping him (forgive the pun) half-assed so he starts to feel uncomfortable from the remnants on his skin causing "itching"....... Oh- if you are a warm cloth mamma- don't use the change in cloth temp as a ploy for power. He'll just poop and not look to be cleaned up.....
Good luck.