I would disagree. If the kid is getting open stares from adults, the mother might instinctively wish to protect her child from:
a) Perverts, real or imagined, who will attempt to exploit her son for his age, naivete and size
b) His schoolmates, who may crack jokes about his size which makes him feel insecure and goofy - if this is the case it would be no different from explaining the mean behaviour of kids to a child who has big ears, or another large malady.
For example, my girlfriend went through a terribly awkward stage in middle school during which time one breast grew faster than the other, leaving a disproportionate chest. If anything the mother was responsible in this case for addressing the issue that her daughter was undergoing, which in my eyes is no different from the issue of size.
However, if the mother is attempting to explain to the kid what "gift" that he has been endowed with, or any sexual information relating to size, that would be inappropriate and should be avoided. Sex should not have to come into this conversation unless absolutely necessary (i.e. he has been assaulted) as discussing the abnormally large penis of an eleven year-old child, in this context, should be no different from discussing acne or another non-sexual prepubescent issue in order not to alarm him and achieve what should be the mother's ultimate goal: his comfort and security at school. Additionally, the mother may wish to bear in mind that the eleven year-old may simply be achieving sexual maturation at an earlier time than his peers - his penis which seems large now may just pan out to be average. I'd recommend thinking five steps ahead in every direction as this conversation will be undoubtedly sticky, uncomfortable and delicate for all involved.