Hi Aditya,
First of all, the Subreport should not be placed in the Page Header. If you're confident that the Subreport will not span over a page then it's fine. Otherwise, it's best to have it placed in the Report Header.
The Suppression formula for the section that holds the Subreport should look something like this:
Instr({?Prompt_Name}, "%") = 0
Whereas for all the other sections in the Main Report, the suppression formula should be:
Instr({?Prompt_Name}, "%") > 0
Replace Prompt_Name in the above formulas with the Parameter you have created.
-Abhilash