I have heard that potty training for BM is hard with some kids. I have heard all kinds of theorys and one is that they see that as a part of them (since it is solid I guess) and it is a little frightening to see it go down the toilet. Also, is he the least bit constipated? It could be that it is painful.
I worked in a pre-school last year, and most of the little boys would pee in the toilet, but poop in their pants. Don't ask me why.
I suggest being patient, and as someone else said, be very positive if he even tries to go in the toilet, and kinda ignore it for a while if he goes in his pants. I don't mean totally ignore it, just don't make a huge deal out of it. I would sternly say, "poo poo goes in the potty, not our pants", lead him to the bathroom and have him take his clothes off, if it is solid enough, have him put it in the toilet and then have him sit on the toilet a few minutes to "finish". Even if you know he is finished at least he will know he has to sit there even if he puts it in his pants. I would have him do the cleaning up to the best of his ability and then just have him wash his hands and go to the bath. I would also have him go "try" to potty like every 30 minutes during that two hour period. I would give him a book (I really like Everybody Poops, or there is another one that is red with a little boy named Joshua in it that I used for my son). Have him sit on the potty and read or get a small toy he can hold in his hands and only let him have it on the potty. It should not be a negative experience on the potty or he will never want to do it. If he gets even the smallest bit out, make such a huge deal out of it that he will want to do it again to get the same response. I would give him a reward too. Even if it is a skittle or M & M or whatever. It should be only given at that time too so it will be a desired item. I know it sounds crazy, but trust me, those little rewards work.
Good luck!