If I should let my son use on will he be able to assess my pass word manger?

Hi gareyray
 
Welcome to the Community Forums.
 
Whilst that is indeed the case so running/having separate Password Manager accounts is the way to go. And to do so is easy.
 
You will most likely want to have main Console access and control the whole thing, but you can also provide a separate Password Manager account to your son (and others in the future).  To do that:
 

1. Log into your Account Conole
2. At the upper right, click your email address
3. Select 'Manage Users'
4. Check how many user profiles you have already set up here.

I presume that it will currently just be one, or have you already set up one for your son?  With a muilti device/user license, you can have up to the same number of user profiles on that same key code.
 
 If you have only one user profile set up, click the 'Create New Users' button.  Each user profile is, just like yours, a specific email address.  Create an additional user for your son.
 
For his new user profile, once created, go to 'Permissions'.  There you can allow him Console access, or block him from it. Also, you will need to set the permissions for Passwords, and if you so desire you can also allow him separate Backup &Sync access as well, in which case he will go into a different Backup & Sync folder and so your son will not be able to access your files or passwords, AND vice versa.
 
Just a quick note here to advise that, the WSA Backup size allocation is for the license/group and NOT per user. 
 
Further, suggestion; that you do not provide Admin Console Access to anyone other than yourself, as that will give them access to make changes (perhaps more pertinent if you want to allow further users to make use of these features).
 
Well, I hope that answers the question/makes sense?  Please do not hesitate to come back here and let us know how you get on or if you have further questions.
 
Regards, Baldrick