I don't own that one, but would start by looking in "My Documents", "My Games" and see if there is a sub-folder for it. If there is you can copy that away and keep it.
Buy the game for your son and install it
Now make a copy of that brand-new gamesave folder (there won't be much in it) and store it somewhere safe.
Try to copy the original game folder to the same place on your son's PC. Fire up the game and see if the saves are there.

If not, look at what was in the vanilla savegame folder . You may need to keep some files from that and just add in the saves

Hard to say without doing it myself. But this approach would work "back in the day"

Good luck!

https://www.ubisoft.com/en-us/help/avatar-frontiers-of-pandora/gameplay/article/managing-your-saved-games-in-avatar-frontiers-of-pandora/000106032

thanks all you all amazing this is a great help