The answer to it does not need to be clear, and for a 6 year old boy with limitied capabilities to express himself it may be even harder. The best one to give you an answer may be a doctor who examine him carefully.
Possible reasons include physical issues like constipation or psychological issues. If the reason is a physical one training together with medication may be helpful, with psychological issues behavior or cognitive therapy may be helpful.
I do not know for sure why I pooped my pants when I was 6, but it was a psychological reason and happened in some situations; I think it was a mean to get attention and care from my parents making rewards, yellings etc (behavioristic teaching method) more of an amplifying effect rather than helping it.
The most common reason for encopresis in a child of six is chronic constipation. However, the situation invites evaluation, first by the pediatrician, then by a pediatric gastroenterologist. Almost always, when a child this young is treated medically (don't take this on yourself) with a regimen (it might go on for a couple of months) of laxative and stool softener the situation resolves.