Under Hindu Succession Act in India - What are the documents required to be executed to disown an NRI son?

There is no such thing as disowning the son under the Hindu Succession Act,1956.

You can not disown any child born to you or for whom you are natural parent, the maximum you can do is disinherit such child from your self owned property only and for that you can make a Will, wherein you make someone else the beneficiary of your self acquired property/cash etc. as the case may be, you may even register such Will with Sub-Registrar of your district and if you so desire publish a newspaper public notice wherein you mention the fact of disinheriting your son and making a Will with regard to your property in favour of someone else.

The property which is ancestral or copacenary in nature cannot be included in the Will made by anyone it has to be the self acquired property of the testator.

It shall be best to inform your son about your desire to do this before your proceed and workout your Will etc.

Why bother - just leave you money to someone else.