Is it normal to be scared of my boss?

100% normal, you are supposed to feel like that if you work hourly. Just learn to play the game. Have some work set aside so when he is walking up you quit chilling and start doing that work. When he leaves go back to chilling.

Look for another job asap. No one should work in fear. Either that or tell him he scares you and see what he does. It's my guess he's the bully type that likes to scare people so don't expect much.