Don't let nothing get near its cage. lol I've never had any snake of any kind start acting defensive all the sudden when it was quite tame before. That's kind of strange but nothing to be worried about. You can try putting something in there he can hide under, when he's under it he won't be afraid of anything moving near the cage. Hide places are good to provide in all cases especially if the snake is in a living room or where there's a lot of movement going on. As he gets older this will more likely go away.
-----
Muhammad Ali