His mother will have to give you medical power of attorney. Will he still be covered by the insurance he has now ? Is there a deductible and co/pay associated with his doctor visits and prescriptions ? What kind of job will you have ? Can you support yourself and him ? What will he do about school ? You will also need legal.power of attorney for things like school.
Carl has a valid pont about federal and state laws. Medical providers and schools are mandatory reporters for possible child abuse. It will not matter that he has his mother's permission to be with you or that he is with you willingly.

