actually, in response to one poster who said that they are being too nice: the only thing this mother is doing right is telling him that it's ok and trying not to make a big deal about it in front of him. adding stress and shame is absolutely not going to help the boy or the situation. other than that, as people have said (and I just want to reiterate b/c it sounds like you really want a quantity of answers, maybe so you have ammunition to take to the mom?), there are really only two reasons for a child to regress to pooping in his pants at this age: psychological and medical. both of these reasons REQUIRE an immediate trip to the pediatrician.
your first line of defense is talking directly to the mom. try to be gentle and empathetic. she may be scared and embarrassed herself, and not willing to face how dangerous/serious this could actually be. she may feel like the pooping is a reflection on her parenting when in fact it might be totally physical. she needs to know that finding an answer will be a relief, and will not result in blame being shifted to her (even if it's psychological; unless she's actually abusing the boy, of course). and she needs to understand that, no matter what, this kind of regression needs to be addressed by a professional.
if the mother denies, or blows you off, or otherwise indicates that she's not going to take the boy to his doctor, then your only other option is to write a letter to the pediatrician, as someone else suggested. you can't talk directly to the pedi; he's prevented from talking to you by privacy laws. but a written letter from you will find its way into the boy's file and the next time the doc sees him, the doc can be proactive about talking about it and starting to look into it, even if she never brings him in for any reason other than the annual check up. just list out the facts, what you've seen, and note that the mother is not trying to get help for the condition. let the doctor draw his or her own conclusions and make his or her own decisions about what actions to take (or not to take); your letter is for the purpose of informing the child's doctor of a medical (or potentially psychological) situation. beyond that, let the professionals do their jobs.
and I should be clear: this step should be taken if and only if you talk to the boy's mother, tell her your express concerns, and she chooses to dismiss you and/or the situation. if she's willing to take the boy to the doctor herself, then there's no reason to get your nose into somebody else's business. but if she won't, then you do need to do something. it's actually a really big deal if this goes untreated.
good luck.