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.