Change payees at mother's request

My son is 15 yrs old with autism. We lived in an apartment his entire life until recently when I purchased a home. It was hard trying to get him to become accustomed to his new bedroom, so I changed payees and allowed him to move in with my aunt who still lives at the apartment complex. It was hard to change his surrounding. Now, I am about to relocate out of state with my job...as a mother & not the payee, I will feel more comfortable if my mother will be taking care of him while I am out of state. As the parent & not the payee, how do I change the payee from his aunt to his grandmother?