1.) Raisens have lots of iron and that tends to bulk you up, so I don't know, but that sounds like it would have the opposite effect you are looking for. I used flax seed in a yogurt and a handful of frosted shredded mini wheats. That worked like a charm.
2.) Are you using big boy underwear? If you are using pull-ups he will continue to poop in them and won't go on the potty for a very long time.
3.) If he goes poop in his pants make him clean it up. Help him take the wipes and clean his own bottome and put them in a plastic bag...then help him rinse out his underwear. the whole time comment on how disgusting it is to poop in your pants and how much better it is to go on the potty.
4.) I watched mine for the signs they were going...You don't get much done watching them so closely...and then I held them on the potty while I hug and kiss their face. I tell them it will be fine. When they finally go, I give them two or three chocolate chips. I chear and go wild...high fives and the whole bit.
5.) And though I've never had any of the four kids I trained to refuse to go poop on the potty after I got them through that first fearful situation....I would punish mine if they did that. Once I know they understand what it means to poop (because they go hide to poop or run from me because they have pooped already and they know they did) then I explain to them that they will get punished if they do it again. I guess I'm a mean mommy, but that's the way my mom did it, her friends did it, my grandmothers did it, my aunts did it and so forth. None of my aunts, uncles, cousins, or my siblings was traumatized by it. We all go to the bathroom just fine. Never suffered a single issue because our moms insisted that we go poop on the potty and then tore our little butts up if we defied them (my middle brother got a spanking once or twice for pooping his pants while playing...he's a computer science and math major. He heads a major department for the state. I think he's fine)
Hope some of that helps.