When someone is incapacitated, how can another person get POA for them?
If your nephew is unable to communicate, it is too late to make a power of attorney, and guardianship and/or conservatorship will be needed. (Unless your state has an alternative procedure that lets family members provide health care consents for an incapacitated person.) This is a situation where you'll have to have legal assistance. (Some states have 'do it yourself' guardianship structures.) See a lawyer in the state where your nephew lives.