The potty training dilema

If anyone can provide some insight, it would be greatly appreciated.

Our 3-year-old has been potty trained since he was about 2.  He showed interest early so we started training him just before he was 2 1/2.  Now he is one month shy of being 3 1/2 and we're having issues with him pooping in his underwear daily. He used to go just fine on the toilet. Now he is obviously choosing to poop in his pants. At first, we just thought he was caught up in his play time or watching a show/movie that he wouldn't even think about going to the bathroom.  Now, he will run to his room and poop in his pants without mentioning that he has to poop.  This has been happening for the last few months and I am so over it. We have tried rewarding him with candy whenever he goes on the toilet, but it has been so inconsistent I just don't know what to do. I have tried taking toys away, throwing away his soiled underwear, and nothing is changing!

To add to that, our 2-year-old is training too. She will tell us every once in a while when she has to go potty. She's in pull ups already and a few times she has actually woken up after 12 or 13 hours with a dry diaper. She has pooped in the toilet once! I am not rushing it with her because I know it will take time and she only turned 2 in February.

Any help/ideas to get him to start pooping on the toilet again would be appreciated!