What great questions. You might want to start off that way.
The first question, well, you could just give him the biological and/or religious response that you believe -- ie, you're a human because your dad and I are humans and we passed down our genetic code to you (in whatever detail is appropriate for him). I'm sure there's books out there that broach this subject.
As to the second question, you can tell him that great thinkers throughout time have been asking those questions, so he's not alone. Ask him if he's come up with any answers to his questions, and go from there. You don't want to go deeper than he's ready for, but you don't want to have a shallower response than he needs. A sort of existencial angst is common for gifted people, I think. So, it could be a jumping off point to talking about religion and/or philosophy -- or it's possible that he's looking for a simpler resonse: "The reason you are here is because daddy and I loved you so much we created a new person. What I want for you is for you to be happy, healthy, and good. Your job right now is to learn -- to learn information, to learn to treat people right, etc. And when you get older, you will get to decide what you want to do." kind of thing.
It is great that he's sharing these thoughts -- which might seem odd and even disturbing to him -- with you. You should encourage him to continue being so earnest with you by acting unphased at his questions/concerns. If you do so, this could be an ongoing conversation where the two of you gradually delve deeper into what it means to be human, what duties people have to each other, etc., etc. Good luck!
