I think Phil's got it here. In the SQL Command window of your OLE DB Source (I assume you're using an OLE DB source since you're reading the data out of the table), after the EXECUTE statement, add a WITH RESULT SETS clause, like this:

EXECUTE dbo.myStoredProc WITH RESULT SETS ((col1 int, col2 varchar(20), col3 datetime))

Inside the double parentheses of the WITH RESULT SETS clause, list the columns in the results of your stored proc and their data types, just as if you were creating a table. Now, SSIS gets the metadata for the result set from this definition rather than trying to tease it out of the stored proc.