Try searching past Mamasource postings. This topic comes up a LOT and I know it's been discussed before so there is probably good advice in the archives, and I know you'll get more today about readiness, etc. My thoughts: (1) is he on the regular toilet or a self-contained small-kid potty? Some boys are OK with peeing into the big toilet because they can stand up to do it and it seems fun, but sitting to poop is sometimes a bit scary (they fear falling in, there's that "bare" sensation that's cold to them because they're used to pooping in a close, warm diaper, etc.)
(2) Make the potty MUCH more interesting than cleaning up his messy pants or diaper. He should be helping you clean up when he poops in his pants during the day. Don't act like it's a punishment but treat it calmly and firmly without fussing at him, like it's "something we just have to do each time, and you have to help." Then have on hand new, interesting books he can only see when he's sitting down on the potty--no other time, and keep up a supply of new ones to introduce the moment the current one isn't interesting enough to make him want to sit a while. Don't scold if he sits a while and doesn't poop; eventually he will.
Remember, at his age, sitting to poop may be both a bit scary and it interrupts his play--he can probably get his business done faster by squatting in a corner and thinks he'll then go on with his activities, but if you make cleaning up something he must do each time, and you make the potty time more interesting, he eventually will come around. Eventually. It may take a long while yet -- don't expect full and perfect training for a while. Be aware that you may even encounter setbacks on peeing and staying dry during the day, especially if there are changes, or his body changes, or he wants more control of his life -- for instance, if he gets sick, his brother gets sick (and gets more of your attention), he starts preschool, you move house, he has a growth spurt...be aware that kids do have these setbacks and don't punish him for them. Praise him a LOT for doing well and praise him often. Good luck.