Until recently, once an address had been used as an Apple ID you could remove it from that ID and use something else, but you couldn't then re-use it as a new Apple ID. It's been reported that this has changed and you can now use any (non-Apple) address as an ID provided it works and isn't currently associated with an Apple ID.
Unless someone who has done it can confirm this your only option is to try it and see what happens. The first step shouldn't be a problem - as you know you can change the current ID/login to another address (as long as it isn't an @mac.com, @me.com or @icloud.com address). Having removed the current address you can then try changing your son's ID to using it. If it doesn't work, he will have to use another address - as it's only used as a login he can create a free one and use that, it won't impinge on any other use.
In either case it's vital to follow this procedure for changing the ID:
- Firstly, if you have 'Find My iPhone/iPad/iMac' enabled on any of your devices, turn it off.
- Create a new email address, for example at Yahoo or Gmail, or anywhere convenient (or you can use an existing address as long as it is not associated with an Apple ID).
- Go to http://appleid.apple.com and click 'Manage your Apple ID'. Sign in with the current ID.
- Where it says 'Apple ID and primary email address' and gives your current ID email address, click 'edit'.
- Enter your new address and click 'Save changes'.
- Now you will need to go to each of your devices and sign out in System Preferences (or Settings)>iCloud - 'Sign out' on a Mac, 'Delete this account' on an iOS device (this will not delete the account from the server).
- Then sign back in with your new ID. Your iCloud data will disappear from your devices when you sign out, but reappear when you sign back in.
I re-iterate: before you start, turn off 'Find My Mac' (or whatever) or you will need the services of Support.