You don't have power of attorney "over" your aunt. You have a power of attorney "from" your aunt. Assuming that the POA is valid, and assuming that your aunt is mentally incapacitated, you would use the POA to file an eviction lawsuit in Justice of the Peace Court after you had given the great grandson a written notice to move out. I assume that there is no lease and he pays no rent. Find a lawyer that does evictions. If your aunt is not mentally incapacitated then she would have to file the eviction herself.
DISCLAIMER: This is not specific legal advice and does not establish an attorney/client relationship.
You need a lawyer that handles eviction cases for landlords. A small business lawyer would also work. YOu can probably find one using the "Find a Lawyer" tab above.