In the event of your flat being under a Society, and assuming your wife has not made any nomination to the society with reference to transmission of her share of ownership , then you should do the following
a) Since the flat is jointly owned by you and your wife, subsequent to the demise of your wife, her share will devolve in equal shares with your son and yourself.
b) Your son can execute a power of attorney in your favour by executing a power of attorney in the nearest Indian embassy
c) Upon receipt of the power of attorney , you as a power of attorney holder should get the power of attorney registered with the sub-registrar and also make an application for membership on behalf of your son for inclusion as a joint owner of the flat by paying Rs100 Entrance Fee
d) You should then execute the undertaking and indemnity (as power of attorney holder of your son) as per bye laws of your society
( Appendix 4, 17, 19, are in general common to all societies as all societies in Mumbai have same bye laws and these are pre- printed formats)
e) Enclose the Attested Xerox copy of the Death Certificate of the deceased member and Xerox copy of the Share Certificate of the deceased member, with undertaking on the Xerox copy, that original share certificate will be produced by the member, as and when asked by the Secretary for making the name change by the Society
(f) CHS will display the notice in its notice board, send a copy to every member of the Society and will publish it in two local news papers having wide publicity, one in local language and one in English.
(g) Any claim from the public has to come within a period of 15 days from the date of publication of the notice. [ under Bye-law no.35 ]
(h) Once there is no objection from the society, you can sell the flat and society will also transfer the share of yourself and your son to the buyer.
Hope this information is useful.