Almost 6 year old not toilet trained

[OCStepMom]

My stepson will be 6 at the beginning of February.  I have attempted to potty train him since he was 2 1/2 with great success, but he would return to his mother's and come back having continual accidents.  Over the summer, I started a reward chart and he did not have an accident for 2 full weeks.  Then he went with his mom for a week and came back having accidents again.  No peeing accidents, only poo.  The frequency has grown and he sometimes has up to 4 accidents a day.  I'm not saying it's her fault, it's just that we've tried everything and all he says is "He's sorry".  We have heard not to punish children for this, etc., and don't, but it sure is frustrating, knowing he is well aware of what he needs to do.  He has quality examples, as my husband and I have 2 other children together that are 3 & 4 and are fully potty trained.  At his other home, he has 5 & 7 year old sibilings that are also fully potty trained.  He's in kindergarten and after school care and essentially, because it's a requirement the child be potty trained, they leave him in it (which my husband will be discussing with them shortly, as poor clean up is creating a rash).  He does not wear a diaper to bed, hasn't for several months and does not wet the bed either.  Any help is appreciated!