It sounds to me that he is putting on an everything-is-normal face for you with the excited talk of the future but this is how he really feels. I'd try to get him to go to a psychiatrist (so he can be prescribed an antidepressant if he needs it) and let him know that he can go in without you and keep it private, as obviously he doesn't want to talk to you about these feelings. If he knows it is just between him and the psychiatrist then he may be more willing. If he won't go for that maybe at least try taking him to his pediatrician for a trial of an antidepressant just to see how it helps his mood. He may not even need medication but just someone to talk things out. There are also teen suicide lines where then can just talk. He seem like he wants to deal with this privately (to the point of denial) so maybe you can help him get private help.