ExecuteReader is a little funky when dealing with output parameters. The output parameters are available only AFTER the recordset is used and closed. From MSDN:
Using a DataReader to Process the RETURN Value and OUTPUT Parameters
This same query can be executed directly with the Command.ExecuteReader (or ExecuteNonQuery if you don't have a rowset to process), but there are several other steps you'll need to take to process the returned values. Remember, you'll have to complete processing for all rowsets before attempting to capture the Return value or OUTPUT parameters. The following code shows how to use the ExecuteReader and a loop to process the rowsets, and then capture the Return value and OUTPUT parameters. You'll find that OUTPUT parameters (even a lot of them) can be handled far faster than even a single row of data returned by a SELECT
You may be better off using a dataset instead - the output parameters handling is more straightforward.
http://msdn.microsoft.com/en-us/library/ms971497.aspx#gazoutas_topic4
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