What security vulnerabilities are there with IDENTITY_INSERT and DBCC CHECKIDENT
DBCC CHECKIDENT


  Caller must own the schema that contains the table, or be a member of the sysadmin fixed server role, the db_owner fixed database role, or the db_ddladmin fixed database role.


MSDN DBCC CHECKIDENT reference

IDENTITY_INSERT


  User must own the table or have ALTER permission on the table.


MSDN Set IDENTITY_INSERT reference



When a user can delete, update, insert, and select records, you are granting the user the ability to manipulate the data within the table.

When a user has alter, they can adjust the columns, data types, seeding information, and much more. Best practice dictates lowest amount of permission necessary to achieve a function so that there is less opportunity that can be exploited. 

Now if you can already modify the table, I'd be interested to see how he locked the permissions down so you could not run at least the IDENTITY_INSERT commands. 

The least amount of permission you need to achieve these goals is DB_DDLADMIN which can open some security implications across the database server. You would also need ALTER TABLE privileges, this is less powerful than DB_DDLADMIN since it can be isolated to a single table and not server wide, but not without potential vulnerabilities.