So the parts I thought would scare him, didn't. Quai-Gon, Obi-Wan, and Jar-Jar almost eaten by a huge fish? No big deal. "They should have gotten their swords out!" But the mere sight of Watto or Sebulba*, even when they were doing nothing in particular, was scary.
Since this is meant to be about children and Star Wars, not just my son and Star Wars, I'll mention that my colleague reports that her daughter, who I think is about seven, is scared silly by Star Wars. Also that I don't remember being scared by any of Episode 4, ever, even though I was not yet six and easily scared by movies. That includes when Luke comes back home from...wherever he went and all that was left of his parent-substitutes were two smoking skeletons who had clearly been slaughtered while trying to run away.
*In Mayan mythology, Xibalba (with "x" pronounced like "sh") is the name of the underworld.