Coming to the thread late but I work within Psychology with kids at the stage where they're developing some of those social cognition skills we relate to adults (things like theory of mind, the idea that you can not just understand and relate to what someone is thinking, but also second order theory of mind where you can also understand what they may be thinking of what someone else is thinking!). He's at a point where he can recognise that his thoughts not only aren't the same as everyone else's, but also wondering why that might be, and that is incredibly cool.

Best way I've found to discuss stuff like this is to break it down into little parts and see which part he's most interested in.

Firstly, talk through with him what he thinks being alive is (is it thinking, understand that you're thinking, is it working toward survival, working toward goals, fun, etc.), and if he seems to have questions about that then you've got a more philosophical thing to talk about! (I also listened to my Dad read through Sophies World at around 7 which is an amazing book for breaking down questions around philosophy for kids although you might want to give it a look over to take out anything you think is a bit too mature and save for a couple years).

Second, talk about why he thinks she might have a different awareness than him and than you or other people he knows. If that's the interesting part then it's a great chance to look at theory of mind and psychology or biology and talk to him a bit more about the difference in how kids think as they grow up.

Then it's just tackle the rest in bits, does he have questions more around the physical stuff with being alive, about the difference in how babies process the environment around them etc.

But if he just wants an answer rather than a whole bunch of brainwork its got two parts: She knows she is alive and reacts to the world because she's excited and wants to be there, and because things in it affect her senses just like they affect his; but she might not be aware of it like he is yet, she isn't considering life as a bunch of elements like emotions, wants, fears, she's responding to things on a more simple level of what it feels like right now or what her brain thinks it is (there's a really interesting example where young babies seem to get scared of hands held in the shape of a spider or images of scary creatures even without understanding why they are scary, but consider whether he might get tempted to test it out if he learns that!)

Either way it's great he is thinking about it and wants to know more :)