K.,
I'm not sure if this is going to be of any help but my son (now almost 8) had diaper issues that we dealt with on and off for a while. He didn't smear his poop but held it and eventually went in his underwear. His sister was born when he was 4. He pooped in his underwear even though he was potty trained at about 2 years old and did that despite our explanations, asking, taking away privilages, etc. Now, in retrospect and after a conversation with a psychologist, I can see the correlation between his behavior and the birth of another child. I remember myself telling him that he couldn't do certain things because his sister was going to sleep, or because I was going to feed her, or because of another thing concerning his sister. I yelled at him when he woke her or played too loud, and so on. Now, I realize that his time with me was suddenly greatly reduced, our expectations of him became very high, and our dissatisfaction with some of his behavior more vocal. Overall, he felt left out, sad and disappointed and never even said that. I didn't spend enough time one on one with him and yelling and taking away his favorite toys didn't help because those were not the things he really cared about. His behavior outside home was perfect, no complaints. The psychologist said that if problems occur at home, then the cause of the problem is most probably at home too. So, I tried to make changes in our family dynamics and spent more time with him, changed some of our responses to his behavior and in time we saw an improvement. With my third child, I made sure that I spent a lot of time with the older kids and included them in everything that had to do with the new baby (taking safety and age appropriateness into consideration). I asked for help, let them hold her, praised them for being big brother and sister, asked them to teach her appropriate behavior, etc. And it worked so much better.
I know how frustrating it is, especially when you get angry cleaning it all up but anger doesn't help (it always made things worse in our home). Your son may be trying to get more attention. Hope you find the solution that is going to help you and your son feel better and function better as a family. Hope it helps.
J. K.