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. (Believe me, there are many "unwanted" challenges in life far worse than reading the Tocacha.) I have taught Bar Mitzvah lessons in the past, and I insisted that in addition to knowing the trope, the Bar Mitzvah boy understand as much of the parsha as possible. It's not a halachic requirement, but it helps with learning the trope, and it gives the boy an appreciation of what his "special parsha" is saying - it also helps for his Bar Mitzvah drasha :-)
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.
I explain to Bar Mitzvah boys as well as adults - consider the tochacha a "blessing", not a "curse". It is as if your father said to you, "Don't go into the tiger's den at the zoo. I know the tiger looks cute and gentle, but if you go there, the tiger will tear your limbs, gnaw at your hands, and so on - your father gives you a list of 20 things that the tiger would do to you."
Why doesn't your father just say, "Don't go there because the tiger will kill you?" Because he's trying to scare you, somewhat, so that it doesn't enter your mind to even THINK of doing it.
The purpose of the Tochacha is that the "father" is the great father, G-d. He says, essentially, "I'm warning you not to go after idols, because these are all the terrible things that can happen if you do." This, way, if you're scared, the idea won't enter your mind to even try it.
So, just like you would thank your dad for giving you the warning, in advance, consider it a blessing that G-d warns you in advance not to do something "dangerous" or detrimental!