I am sure you have done the same thing but I googled hoverboard randomly stop and this came up.
https://www.streetsaw.com/pages/hove...-off-by-itself On the page it says that the problem is from the battery circuit board and sounds like it is a safety feature that the board shuts down instead of causing damage to the board.
If this is the same cause for what happened to your Son it might not be a defect and just a very poorly planned safety feature.