I don't think we have anyone who checks this forum who actually lives in the Alpes-Maritimes, let alone France, to search for you.
The place your uncle was living at, 67 Chemin Petit du Four, is a small apartment building called Les Alpes. You can look at it on street view on Yahoo maps or GoogleEarth. I'd say at his great age, your uncle probably couldn't manage living there on his own anymore and is either living elsewhere or has passed. I think they would have tried to forward your mail if he was still around. But you never know.
I think the best place to start would be to call or write the police. They should be able to help or point you in the direction to someone who can.
To phone Antibes dial 011 + 33 + the Local Number (without initial zero). So to call the police below you'd dial
011 33 492 907800
Commissariat de Police Commissariat Central
5 Avenue Frères Olivier
Juan les Pins
Telephone: 04 92 90 78 00
I managed to find the phone numbers of three people who live at Les Alpes. You could try calling or writing them to find out if they know what happened to him.
Gijsbert Bos: 09 53 80 50 96
Jacqueline Fleury: 04 93 33 29 59
Laurent Labinjan: 04 93 67 61 32
Best of luck and let us know if you find out what happened to him.