You can't specify this permission in one statement. You need to grant this permission in each seperate (user-)database.

Set the focus to the user database and create the user (if it doesn't exist yet). Execute the command "EXEC sp_addrolemember 'db_datareader', '{login_name}'" to grant the read-only permission. Execute this for each user-database.

If you add this permission to the [model] database, the permissions will be inherited to every new database created.