2 accounts, 1 computer

Me and my son share the same PC. I already have GW trilogy installed, on my own user account. But I want my son to have his own copy of the game
(I want him to have his own character slots, storage, etc).

If I purchase another copy of GW trilogy, will I have to re-download all the content for my son's account? Or will it use the default content that's already on the computer? Thanks.

You should be able to just create an account/log in with your existing GW client. You won't need a new set of files.

yes, you wont have any issues, just remember to log in with your own email, pass and character name (and your son his)...I have multiple accounts and play them all on the same computer (not at the same time!), so it is easy to do.

If you don't plan to play at the same time, your son can just set up an account and then enter his username, password, & character name into the same installation you use. In this scenario you can think of the GW install as being like a browser - that is, anyone can use your browser, but they enter their own username and password to log into these forums, for example.

If you plan (or start) to play at the same time, you can either use some form of multi-launcher (which can cause some confusion with regard to controls) or you can get him his own computer to play on (doesn't need to be fancy).
In either case you only need to copy your installation to the other folder or computer - there's no need to re-download it.