A Google will reveal charities that accept old PC's to refurbish.
As for the data issue- It depends on how concerned
your son is about the content.
a Reformat and reinstall the O/S is probably the best first level.
The many disk clearing programs on offer probably have as many data recovery programs to defeat 'em.
The only sure way of data not falling into the wrong hands is to remove the disk and destroy it. Then there is not much point in trying to find s new home for whats left.
Bear in mind also that if the charity you select is far from your base- getting it there in person or by carrier can be costly.