You can't share media between two accounts on the same computer. The way people get around this is to put the media in a place where both accounts can access the same media directory. It is not ideal and is not without its own issues but it does mostly work.
Your best bet is to solve the problem you are having with why your son's account can't log in to the iTunes Store since that would allow purchases made in one account to automatically be downloaded to the other the next time they connect. That's a new feature of iTunes in the cloud.
By accident I came across a solution that may work for you. This works on a Mac but I think it should work in Windows as well.
On the Mac, if I log into a user account and launch iTunes and then use Fast User Switching to log into another account leaving iTunes running in the first account. I can then launch iTunes in the second account and access all the media stored in the first account's iTunes library using Home Sharing.
I believe Windows has the same ability that allows one user to log into their account while another user is already logged in. If so you may be able to do the same thing in Windows. I'm not sure what happens in Windows to running application when the second user logs in though. If applications are allowed to continue running then I don't see why this shouldn't work in Windows as well. If applications are paused then I can see this not working. The key here to to not log out of the first account therby forcing the applications to stop running.
The downside is that your son would first have to log into your account and launch iTunes before logging into his own account. If giving your son access to your account password is not ideal you could also create a third computer account that both of you have access to that would be used only to store and buy iTunes content. That account could be used as the sharing account that you both can have access to while logged into your own respective accounts.