How can my son access my iTunes account from his windows 8 'sign on' on my computer?
Apple provides an answer to this question.

In short:


Quit iTunes
Go to \Users*username*\Music\iTunes (where username is your 
Move the iTunes Media (might be iTunes Music) folder to a public folder, e.g. \Users\Public\Public Music
Now, on each Windows account:

Launch iTunes
In the Edit menu, select Preferences
Click the Advanced icon**
Click the Change button
Navigate to \Users\Public\Public Music (or whatever public folder you moved your music to)
Click OK



You should be all set. If you’re not, that might be because iTunes for Windows apparently doesn’t automatically move your music to its iTunes Media folder. In the account which holds the music:


Launch iTunes
In the File menu, select Library > Organize > Consolidate Library. Your music will be copied into the iTunes Media folder. You may want to do a bit of cleanup to avoid data duplication.