I tried to debug my dynamic query via dbms_output but seems like the query string is too long for dbms_output buffer.
I got :
You can Enable DBMS_OUTPUT and set the buffer size. The buffer size can be between 1 and 1,000,000.
dbms_output.enable(buffer_size IN INTEGER DEFAULT 20000);
exec dbms_output.enable(1000000);
Check this
EDIT
As per the comment posted by Frank and Mat, you can also enable it with Null
exec dbms_output.enable(NULL);
buffer_size : Upper limit, in bytes, the amount of buffered information. Setting buffer_size to NULL specifies that there should be no limit. The maximum size is 1,000,000, and the minimum is 2,000 when the user specifies buffer_size (NOT NULL).