A very simple, cheap solution would be a lipo low voltage alarm/buzzer like this:
http://www.ebay.com/itm/1S-6S-LiPo-B...item43aaa3aac4
The fact that you (I mean your son) is riding it would mean you (he) would definitely hear it (not like a rc vehicle at a great distance) - so it should be very reliable.
Anyway, just a thought.