I thought I responded to this once before but I guess not....
It makes no sense that the son is arguing with his own attorney's opinion that no further documentation is necessary. FWIW, I've never heard of revocation papers for a POA and I've acted as and witnessed signings on a lot of them.
As stated above, most POAs and Wills read along the lines of 'as of this date I revoke any previous ___ by me made' and then go on with the new stuff. That is sufficient.