I just went through this exact same thing with my daughter who was 3 in June and also has a baby sister.
We've tried everything for the past 4 months to get her to poop on the potty. I even tried making her clean it up herself, which was a disaster and made me even madder because I had to clean up the entire bathroom and then give her a bath! I even grounded her from going to the park with her Nana. Nothing much worked.
Finally I just gave up and tried to be calm. I told her that her body knows how to do it and asked her why she thinks she can't do it? Turns out that she was scared and didn't really understand what to do. She thought that when she pushed so hard, she would throw up. We finally gave her a stool to put her legs up on in front of the potty and one day when she was in the middle of going in her underwear again, I put her on the potty and talked her through it. It turned out that she needed to learn to push and then BREATHE and then push again. Now she is SO proud of herself she has done it every night by herself for the past week. Try not to worry, your son will get it eventually. Just try to stay calm and if one thing doesn't work, try another. :)