Hi A.,
Well we are at the end of this long road, so here is what I have done and maybe it will help.
First I stopped buying the sippy cups my daughter lost her last one about a month ago and I told her we weren't buying no more. Then we while I told her this and we had no more sippy cups when she wanted some choc milk we told her she could only have a drink and not a cup. I would fill the cup up about 2 inches with choc milk, let her have a drink and she went to bed. Now while all this is going on we have noticed that her pullup has been staying dry and every morning she is waking up dry so I told her when the pullups are all gone we aren't buying anymore and she will sleep in panties we started monday night and so far we are completely dry and no accidents. Now this hasn't happened overnight and it has taken about a month and a half to come this far.
As far as you son goes and not pooping in the potty I wouldn't pressure him but I would do the opposite...he obviouly knows when he has to go potty and he has proven that he can get himself there. So let him have the control but do it in a telling way but not asking. What I meen is when he gets up in the morning tell him go potty, and before/after nap and before bedtime. let him control all the other times. And then when he poops in his underwear let him know he needs to clean them out and here are the reasons 1) you didn't poop in his underwear he did 2) if he is a big boy and he has shown me he is then big boys wouldn't poop in their underwear 3) you're a big kid also and you don't poop in your underwear you go to the potty. A couple of times of cleaning his own underwear as others have said will make him think twice about continueing to do this. Here are my sayings to the kids "if you are big enough to drag it out, you are big enough to put it away" and "if you are big enough to make the mess, you are big enough to clean it up the mess". I hope this helps you W.