Forwarding Mail

Your legal residency in terms of voting rights stays with the last state where you were officially a resident anyway.

Now in Massachusetts when you register for a drivers license they automatically register you to vote. As far as legal residency I have always found it to be where ever you filed your Federal Taxes from.

My drivers lic has my mailing address which by the way can be anywhere you want it to be

Unless you need to have your DL at your stick house this might be true. When I moved to Florida and wanted to claim my resident status for the real estate tax freeze they wanted bills and my DL both with my home address. Same here on Nantucket where I now live when I went to get my 40% primary home tax exemption and then my cheap ferry tickets for running back to America to shop.

Since the OP isn't playing any games trying to avoid anything I would suggest just using their son's address for taxes and voting. If they don't want him to be bothered with forwarding their mail then just use one of the services for that piece.

Dave