5 year old regressing in potty training...?

My 5 year old has been potty trained since before he was 3. He is my stepson and I am the one that trained him (He has slight fetal alcohol syndrome from his birth mother, which make him behind in speech and socialization). He has always been so good at going to the bathroom, even at night! But now all of a sudden he is pooping his pants every day. There has been no change in our home except his 19 month old brother is trying to use the potty. We always make sure to let him know that our little one needs to use the potty and he is so wonderful at showing him how! I don't know what to do to help him use the bathroom again. He thinks it is funny that he poops his pants. I don't think it is attention because he won't tell us when he goes. He just sits in it until we find it and then he says that it's not his fault. I would appreciate any suggestions on how to remedy this problem. Please, no rude remarks as this is a serious problem.