Transfer of flat to son

Obtain their death certificates, and legal heir certificates from the concerned and then make an application with copies of the same and copy of the will to change into his name.

If they had only son the property otherwise would go to him , why would anyone execute a will. Now if you want to transfer the property in terms of will and not through heirship then probate it as in some cities you need to probate the  will otherwise simply submit application for transfer of property to the concerned section with death certificate and legal heir certificate.

If the person is dead, the will becomes effective and needs to go through probate, which is a legal process that validates the will and distributes the property and assets to the beneficiaries.