Hi, I'd like to help you with your question.
It sounds like this might be one of three things.
One, your son is using this as a way to get attention. He finds that when he says these things people focus on him. Kids sometimes act out and really do not mean what they say, but they find what they say works. Their motive is to be the center of attention.
Two, your son is feeling the pressure of being a great kid and this is his outlet. Some children who are high achievers will act out dramatically because of the amount of pressure they feel to perform. That is not to say it is your fault. It isn't. It is the nature of a highly intelligent child to put pressure on themselves. This is often due to their immaturity in handling the amount of pressure they feel. As they grow older, the start to be able to handle the pressure in other healthy ways and this behavior goes away.
Three, your son is depressed and sometimes feels suicidal. This is unlikely, given your description of his otherwise normal behavior, but I need to point it out as a possibility so it can be considered. Normally, a child who is depressed and suicidal would have more symptoms that your son does, and you would notice other behavior that corresponds with depression. But it is an option until it is ruled out.
In order to rule out depression and possible suicidal intentions, it would be a good idea to get your son an evaluation by a therapist. He can see any qualified child therapist at the Master's level (this includes M.Ed, MSW, LSW, or MA level therapists). They can do a full evaluation and determine if your son is depressed or has any intention of hurting himself. Talk to you son's doctor about a referral. Or you can search on line at http://therapists.psychologytoday.com/rms/. If the therapist feels there is anything to be concerned about, they will let you know and recommend treatment.
As long as you get the evaluation to be sure about your son, I would not worry. He would show more symptoms than he is right now if he was depressed and suicidal. And he has a lot of support through you and his family which makes a big difference. With some time, he should mature out of this behavior and find more constructive ways to deal with his feelings.
I hope this has helped you,