Very curious if you made any progress hear since last year. Our son will not pee or poo in the toilet as he also says he is afraid. We have the same thing that he will not go when he is in public which makes things a bit easier at least. We also let gave in to letting him go in his pull up as he would never go and felt this would create more problems. We have a bathroom schedule that includes after meals to be close to when he should go but it has not worked. The comment on waiting until it is too late I do not feel hold true as he just doesn't want to go in the toilet. He made a comment to the effect that he has to remember everything since he was a baby. I am not sure what it meant. I think it means that this is what he is familiar with and it works. Incentives and unincentives do not prove to be any motivation. The only thing I can think of is getting him to do it without realizing it. Have you had any success since last year?
This all sounds very familiar. I handled it by having my son sit on the toilet within 10 minutes after breakfast and lunch (if eaten at home) and also as soon as we came home from being out during the day. If he hadn't had a bowel movement by mid afternoon, he had to sit on the toilet again. I told him he could have a treat (for him it was Pez candy) if he pooped in the toilet. For a long time he would still have accidents in his pants because he wasn't doing the whole job at one time, but after several months he pretty much stopped having accidents and started going on a somewhat predictable schedule. He didn't ever want to go sit on the toilet and yes, I usually struggled with him to get him there. It's not fun.
It's possible that the reason he's scared of doing a bm on the toilet is that when he was holding it & getting constipated it hurt a lot to go. He can't verbalize that to you, but he associates doing bm on the toilet with pain. It's less painful to squat and go in his pants when he feels the urge. He doesn't do it in public because he knows that's socially unacceptable, but he feels safe enough to do it at home. Another thing that may be an issue is that after someone's bowels are impacted if the colon was stretched the nerves & muscles can take a long time to get back to their correct shape and strength, and he actually may not be feeling the urge to go until it is basically too late to get to the toilet. In public he just concentrates on holding it in, but at home he wants to release it for relief.
I wish I knew of a magic cure, believe me I do. Just know you're not alone & he's not doing this to make you angry.