Amazon RDS unable to execute SET GLOBAL command
Amazon does not give you SUPER privileges on an RDS instance (to prevent you from breaking things like replication accidentally).
To configure group_concat_max_len, use an RDS parameter group, which allows you to configure a group of settings to apply to an instance.
With RDS global settings are managed via the ParameterGroup API. You can also edit them from the web console or using the aws command line tools.