given your answer and a review of the thread I am even more confused than ever as to what you really want to achieve.... Anyway if it helps to reboot after inactivity or on other cues, I can recommend
Slawdogs smart shutdown which I have used to implement shutdown from a remote control in the past...
So, my best guess is you want to turn the computer on and walk away and find yourself logged in on your return, but if your son turns on the computer he gets a certain amount of time to log on before it "defaults" to logging you in? If this is right then it would be really helpful to confirm that and maybe someone can think of a way to implement that, but I am unsure - I now the GRUB boot loader will give a timeout before booting - but that is used to select OS rather than the manner and OS boots, perhaps someone can think of a way to bend it to that task though, if logon user might be a parameter you could be the default and your son might need to select the "other" OS with his login before it times out and boots with you logged in....