I am wondering if the OP did not fully describe his case well..for example, if he were suddenly the guardian of this 8 year old, and didnt' understand the immigration process, and wanted to live in Canada, with his 8 yo nephew, to not disrput the child's daily life, I could understand the question better. But with respect to the general undertaking to sponor any someone, the sponor must be an adult.
If the case is similar to what I suggested, perhaps under Humanitarian considerations...the guardian[uncle] would be allowed to stay in Canada for his family member....but again, more information would be needed to understand what the OP was trying to ask.