When you place it in your own system, is it in addition to the HDD already there? If so, Windows will recognise it as extra storeage.
Does son's PC work normally except for the fact you cannot access the HDD manually via "My Computer"?
If so it has to be an OS problem and maybe a Repair Install will sort it.
Apologies if I am teaching you suck eggs, but normally the OS has to be re-installed if a major component like a mobo is changed.