Teach your son that when people are afraid and uncertain about how they're going to feed their families, they become more nationalistic/xenophobic, and that sometimes manifests itself as racism. It's human nature, and it's happened many times in history. Much of Trump's racism is actually rooted in nationalism or protectionism, which isn't exactly the same as, for example, the enslavement of Africans in the US.

Nazi Germany arose the same way, due to the reparations the Germans were forced to pay after WWI. Why didn't it happen again? The Allies didn't force Germany to bankrupt its economy paying reparations after WWII.

Desperate people are dangerous. Due to the global recession and the decline of labor in the US, lots of people are desperate. Trump offers them hope. It's much more nuanced than Palpatine in Star Wars, who was so obviously evil, and it's something that's happened in every democratic society.