Voluntary registration :
A person, though not liable to be registered under section 22 or 24, may get himself registered voluntarily, and all provisions of CGST Act, as are applicable to a registered person, shall apply to such person – section 25(3) of CGST Act.
In my view, voluntary registration does not really mean that he must pay tax even if his turnover is below taxable limits, though harassment from department cannot be ruled out.
He must have income tax PAN – section 25(6) of CGST Act.