Don't know if it's "normal" or not, but my DS10 is the same way. He takes what he needs/wants from whatever at the time, then moves on. He usually goes back to different interests, but those he doesn't, I just assume weren't all that appealing to him.

I personally like that he's got an interest in so many different things because it only broadens his world. If he chooses to go more in depth with something, I certainly wouldn't discourage it, though.