[stumped]

here is one solution
you need to pick ONE pc to be the master copy of the calibre library. do all the edits on that.. INSIDE of that calibre library can be 2 different virtual libraries - yours and his- that is 2 different views.
then you can mirror( one way sync that entire library to a 2nd PC with various free utilities ( I use Free file Sync- mirroring is quick and easy once you have set it up.
having one librarian and one master copy is the safe route. if you both start updating in 2 different places then try to combine stuff its' more likely to go wrong.

another way - just for getting a few books remotely onto his Forma, is to link the Forma to dropbox, then you put copies of the books he want into dropbox and he gets them from dropbox directly onto the forma
the forma has dropbox integration built in. you just tell it the dropbox credentials and let it link, then it creates a special RATUKEN KOBO shared folder in dropbox, You from your PC, can upload (epub format) books into that from your calibre library via a browser or via the dropbox app. Your son, from his Kobo just turns on wifi and can see, read the books

the 2nd way is simpler. otherwise you have to get a bit geeky to set up 2 libraries, and to set up mirroring