Q: Error when changing password

My son was trying to change his user account password and he clicked on the password assistant.  He had already typed in his new password, but thinks the password assistant changed it to something else and he clicked on it without really paying attention to what it said.  So now he doesn't know his user password.  I thought as administrator, I could change his password via System Preferences (under Users & Groups), but his account is grayed out and so not accessible. 

OS X Yosemite 10.10.2

MacBookPro (late 2011) not that this should matter....

When I use the Finder to look at his account, I get the following message:

The folder "Desktop" cannot be opened because you do not have permission to see it's contents.

Can anyone help?