L’kovod the Bar-Mitzvah of Naftali Tzvi Yehudah Baum:
We read (actually, the Bar-Mitzvah boy will read) in this week’s sedra about the ben-sorrer u’moreh. This carries a lesson, says the Sefer HaChinuch. He says that this parshah teaches us that for the first 3 months of adulthood, a person must be even more aware of his new obligation to guard against sin (if you decide to look for it, it’s in mitzvah 248).
Nevertheless, or maybe on the contrary, due to this, a Bar Mitzvah is a most joyous occasion.
It really should require that the Bar Mitzvah boy recite the shehecheyanu berachah in honor of his new acceptance of mitzvohs. It is best if he wear a new garment, and recite the berachah over that, having in mind to refer also to his acceptance of the mitzvohs.