Any way to enable File Vault for just one user?
FileVault 1 used to do just this but the current FileVault 2 is whole disk encryption. It is technically possible to use the Logical Volume Manager underpinning FV2 to accomplish this, but it's rather complicated and not supported by Apple or any GUI System Preferences. 

You could consider using an encrypted disk image created with DiskUtility. This would give you an optionally mountable disk that is encrypted. It appears on the HDD as a large file (like most installer .dmg files) and mounts with a double-click and password. This is similar to what FV1 used to do with a user's home directory.