I distinctly remember being taught the one syllable rule in elementary school in the US. There are, of course, some exceptions, like "narrower" and "quietest," and "stupidest."
To me, "most stupid" sounds hipster or foreign, while stupidest just sounds right - in normal speech. "He is the most stupid" is perhaps more memorable for sounding off: It's natural to emphasize "most".
Is he more stupid or stupider? I would say stupid, stupider, stupidest, red, redder or more red, reddest, and brilliant, more brilliant, most brilliant, but smart, smarter, smartest.
