First off, full disclosure: I don't have an Android and I'm not an Amanita Design developer.
So, that having been said, here's my best shot: It says
here that version 2.0.21 has "
Cloud Save" Maybe your son can figure that bit out?
It also says there on that GP page
!! Please note that your old saves from 1.X version will not work, but you can use the "Lost Save" functionality within the game to partially recover your position (there are 6 pre-saved positions from various parts of the game).
If he can figure out how to use the "Lost Save" feature to return to someplace ~close~ to where he left off on his Nexus 7 your son could take off from there on his new Galaxy S3 and only have to replay a few levels to get back to where he left off. (have him look at the
Walkthrough topic on this forum to get a sense of where he left off)
Either way that's the best I can do for you amigo. Good luck and please update your topic when you figure out how best to transfer your son's game progress to his new device.