Potty training for poop!

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My bonus son is three and a half and we've almost got the pee part down of potty training but the poop - not so much. He's been delayed with all the milestones and really only started talking a few months ago. We know it takes time and we're very patient with him, never give him trouble for accidents or anything like that. The only thing is that he will poop himself and not tell us. I'm not sure what happens at his moms house when he has an accident, I know they have the same issues but the communication isn't strong between his mom and my husband. Anyway, are there any ways to encourage him to tell us when he has an accident? Usually I'll smell it or see it and we just go to the bathroom, clean up, I remind him to tell us when he has an accident and accidents are okay but to try to remember to use the potty. I'll ask him where poop goes and he says the potty, but I don't think he can wrap his head around it yet. Any suggestions? Thanks!!