Granting Edit Permission on Stored Procedures

I have a user on my database that has the following base permissions :

public
db_datareader

I need to give this user permission to edit a single stored procedure. I have tried using the following command :

GRANT ALL ON stored_procedure_name TO username

Which executes successfully, but the user still cannot edit the stored procedure.

If I give the user db_ddladmin permission they can edit all the user stored procedures, but for security reasons I would prefer to be able to this this at procedure level rather than a global permission on all user procs.

Does anybody know how I can do this?

EDIT : This is on SQL 2000Does this user have the same permission on some tables which related to this procedure?sql