Potty training WOES!

Jacobi is 3.5 years old. He's intelligent in so many ways. He's funny, kind, energetic, he's all boy! I've been working with Cobi, to get him potty trained, for quite some time now. It seems like he's trained and then he has another seizure and ends up reverting back to diapers. I'm so frustrated and am not sure how to go about potty training him completely. He knows when he needs to use the toilet. He knows how to go on the toilet. He knows how to "hold it". I'm just not sure how to get him back to using the toilet consistently after he's had another seizure. I don't want to rush him, yet I know he's capable of no longer wearing diapers. I guess I should just jump back in and get him going on the toilet again. It's just so frustrating to see him take 1 step forward and then 4 steps back. The other part of the problem is that he's never had a bowel movement on the toilet. He'll urinate in the toilet great but has never poo'd in the toilet, always in his underwear. I'm thankful he'll still have bowel movements vs. holding it in but I just don't know how to get him to do that in the toilet. He gets a small treat, usually an M&M for urinating and he knows he'll get 2 M&M's for poo but still goes in his underwear. I'm not quite sure how to stop this habit. Lord, give me wisdom to know how to get this child to poo on the pot!!! If you've got any tips I'm all ears! Please share with me how you got your little ones potty trained! Micah is next up and Shaelyn isn't far behind! I'm hoping they'll be easier since they get to watch their big brother. Shaelyn already sits on the little kiddie toilet anytime we're in the bathroom. We'll see how long it takes her to pick it up! Thank you Lord for wisdom!