It sounds like your son has a problem with constipation. First of all, BM's that large hurt and tear his rectum, so he may be trying not to go because of the pain, I went thru this with my grandson. He really can't help this, and should not be punished. What is happening is that he has some impacted stool, and what is getting into his underwear is leakage which he cannot control, it just leaks out. Stool will stay in liquid form and go around the impaction and just seep out. But he will still have the large BMs. No child this age will go in their pants, it's highly embarrassing and the other kids can be cruel. The fact that he hides his underwear says he's embarrssed. Your nephew needs to be evaluated by a pediatric gastroenterologist who will advise you to give him an enema or suppository to loosen this impaction. Then he may want your son on Miralax (stool softener) for awhile. It not habit forming. Keep in mind that too much fiber is as constipating as not enough. Please have the parents stop punishing him and get him help. He is NOT doing this on purpose.
It may be lack of fiber but it's likely that he wasn't toilet trained properly and so doesn't understand the importance of "doing the doo" frequently - and into the toilet! A lot of kids seem to have this problem nowadays...
They can easily put more fiber in his diet with Fiber One or bran fiber flakes on breakfast cereal.