I did a little digging and this might be doable by setting the stylesheet property of a python Toolbox class. You'll have to convert your script tool to a python Toolbox though, if its not already. I haven't done much in python toolboxes so can't tell if this will work though.
defining-a-tool-in-a-python-toolbox.
and for the style sheet, maybe this can help (borrowed from Stack Overflow):
...
Copy "C:\Program Files (x86)\ArcGIS\Desktop10.2\ArcToolbox\Stylesheets\MdDlgContent.xsl" to the same folder as your script. (or somewhere it can be reached)
Edit line 467 to remove STYLE="display:'none';", i.e. change:
<DIV ID="{PropertyGroupName}" STYLE="display:'none';" onclick="window.event.cancelBubble = true;">
to
<DIV ID="{PropertyGroupName}" onclick="window.event.cancelBubble = true;">