When my stepson was in kindergarten he did this as well. He is now seven and it has stopped. We found out he had a condition called incompresis (I don't know if that is the correct spelling.) Any how, there are two types, intentional and unintentional. We took him to his pediatrition and found out he was doing it intentionally, we got him counceling, worked very closely with his teacher and disciplined him consistantly. However, just from what you said about the little boy not being able to "feel" it makes me think that if he does have this condition, it would be unintentional, in that case, the doctor can help. It can happen for all sorts of reasons, stress, nervousness, low self esteem, feeling embarassed, even medical reasons.
Before I took my stepson to the dr. I read a lot about it online. There are some great articles. Maybe you can find some and find out if you think it is worth taking further action. Then you can talk to the mom and give her the articles. I would have loved someone else helping me with the leg work. This is a very hard situation. It is so nice to see there are teachers out there who really care. Thank You!