Oh, dear. I can relate. I was worried about my son not pooping in the potty (he did pee fine for a long time before he was ready for poo in the potty...). However, as much as I was frustrated about having to change his diaper/underpants numerous times, I tried not to force him. I think forcing a child in any way to do something will only serve to encourage regression. Being in control of your bowel movements is a big deal!!
I guess what I would most recommend is to talk to your son about how big boys go poo poo in the potty. Not just his brothers and sisters and other peers, but older kids (maybe a child he looks up to and likes). This worked with my son, because he adores his older friends, and also I showed him his new underpants, and he was excited to wear them. Yes, we had a few accidents here and there, but ultimately, my son was able to decide when he wanted to go poop in the potty, and I always praised him when he at least "tried" and sat on the potty, even if nothing came out.
Also, as my son was also precocious in his understanding, I was able to talk to him as an almost 3 year old, and tell him that I am fine with him staying in diapers as long as he wants to, and mommy is happy to change his diaper as long as he needs me to (even if I wasn't really that happy, I felt it would help him to feel secure in the knowledge that there is unconditional love from me) and that babies wear diapers, but when he is ready to be a big boy, he can start wearing big boy underpants and go poo poo and pee pee in the potty. Well, the conversation helped, and on his third birthday, exactly, he told me he was a big boy, and didn't need diapers anymore. Of course, it took a few months after that to really get the rhythm and quite a few spills, but I made sure to be patient, and keep encouraging him. It really did happen on its own after that, and my son would sometimes talk about how so-and-so was a big kid and didn't wear diapers, and when he finally started poo in the potty, he was so proud, and felt so confident. It's a great feeling when it happens, so I wish you luck - I'm sure your son will come around, when he is ready. XO