Should a Bar Mitzvah--who is reading the rest of the parsha-- read the curses (tochicha) in Ki Tavo? I have heard of some traditions that suggest someone else should read it, but I'm beginning to think it might be a tradition that is no longer around much, much like the "problem" of who to give the aliyah to.
I know of no specific halacha or custom that would prevent a Bar Mitzvah boy from reading the Tochacha. I've seen several B'nai Mitzvha read the entire pasrha Ki Tavo on their Bar Mitzvah Shabbat, and no one thought this being any different from any other Bar Mitzvah Shabbat.
As a Ba'al Kri'ah, I think that if reading the Tochacha is a challenge for the Bar Mitzvah boy who is learning kri'ah for the first time, then I consider this a "positive" and encouraging challenge. Point is - the tochacha is an important part of Torah and its statements do not display any "bad vibes" or "horrible omens" to anyone who reads it or hears it. If anything, the tochacha is a blessing from G-d, since he's warning us, as a father warns his son of impending danger. So, if a Bar Mitzvah boy reads tochacha, in a sense, it should be considered a special honor!