It can take a LONG time for a child, especially a boy, to poop on the toliet. I counseled a child once that had huge issues with pooping - he would hold it in for DAYS, sometimes to the point where it would make him sick. All his family thought about 24 hours a day was his bowel movements - Mom was anxious, nervous, and at her whits end. After much assesment, the root of this childs non-pooing issues was control. No matter what anyone wanted him to do, HE had the control over his own bowels. Everyone wanted him to poop so badly, and he was the center of attention (regardless that it was negative attention, it was still attention). He had, and still has, other behavioral issues but it seems that little boys in particular pull this I wont poop in the toliet business.
Since it seems that you have exhausted most ideas, here is what I suggest: when he drinks his milk at night or in the morning, put 1-2 teaspoons (dosage depending on how big he is) of milk of magnesia in it. You can get the non-flavored type, or even mint flavor in chocolate milk (the chocolate milk always works!). Since milk of magnesia is a very gentle stool softener, it will soften his bowels over night or in 4-6 hours, and he will have no choice but to poop. It will not cause him to have diarreah (he will if the dosage is too high) and the urge to poop will be sudden - it will not give him a chance to use a diaper, so run him to the bathroom and put him on the toliet. He will have no choice but to go - unless he is one of those stubborn children that will poop his pants before going to the potty. This method is worth a try, though. It will not hurt him, and may solve some of his pooping issues.
Try getting him some special "bathroom only" materials: buy a few fun kids magazines, special soap for washing his hands and flushable wipes (Kandoo is carried in most grocery stores, and makes a line of soap, flushable wipes and a few other things to help encourage children to go the bathroom). After he is done and has wiped and washed hands, let him spray a little room spray in the bathroom to make it smell good. The point is, try making the bathroom fun with bathroom only rewards - we want him to enjoy being in there, and for him to get some positive feeling after he has done his business. You can even reward him with something special afterwards of your choice to give him even more encouragement.
Hope this helps - feel free to send me a message if you need more ideas!