We had some of the same problem, except our DS didn't hold his poo for that long. He is 4 1/2. After having been almost completely potty trained he started pooing his pants, and also tried to hold it in, so that he had some quite big and painful poos as a result of it. It was a bit of a vicious circle, since the dislike of the toilet makes him hold it in, which makes it more painful to poo, which makes him dislike the toilet more.
We have basically tried everything, except medication. He never kept it in more than a day, so it didn't affect him as much as you describe that it affects your poor boy and your family. It sounds really awful.
It was still stressful that pooing took up so much of our daily life. Even though it was a long time since he had had a painful poo, he had such aversion against the toilet that he would prefer to poo in his pants after holding it in for as long as possible. A lot of the day would be spent trying to hold it in, and he would avoid being with other children while doing this, so it affected his social life. It was also very embarrassing for him when other children discovered that he had pooed his pants.
We tried to encourage toilet habits, we tried to give him control, we tried rewards (which goes against our parenting beliefs), nothing had any effect.
In the end I began to suspect that he finds the toilet itself uncomfortable too, not just the pooing. He is very sensitive to all kinds of physical discomfort. I asked him if he wanted a special, extra comfy toilet seat, but he said that he wanted a potty! I never thought he needed one, my other kids have always just used the toilet and haven't had any problems with potty learning. I always thought it was really simple...
We went to a baby shop and let him pick out the biggest potty they had, one of these: http://www.kidsandcare.no/index.cfm?...2&b_kid=490237
I tried to stay serious when he showed his enthusiasm in the shop, and I am sure the woman who sold it to us did too, as he is tall for his age and looks like he is 6 years old - and so excited about getting a potty!
It has actually worked really well, even though he still rarely says that he needs to go, he doesn't mind trying when we see signs that there is something about to happen. We have had many days without pooey pants, and he is really happy and proud every time he poos in the potty. He likes that it can be brought to different rooms, so that he doesn't have to sit in the boring bathroom to poo. He likes pouring the poo in the toilet and even insists on cleaning the potty himself... it's just like a potty learning phase all over again, in that he is really interested in this body function instead of trying to avoid it.
I'm sorry this turned out so long! And I don't know if any of this is any use to you. I hope you figure something out, or it sorts itself out soon!