Does a Bar-Mitzvah boy recite a Shehecheyanu when he becomes a Bar-Mitzvah? It seems some Poskim feel it should be said the first time Tefillin are put on. But why not on the actual change of status – now every moment is an opportunity to do a Mitzvah for the first time?
Also, if saying a Shehecheyanu over Tefillin (or over a garment before Tefillin) is the Bracha said when Tefillin are first put on (often a month before the Bar-Mitzvah) or on the day of the Bar-Mitzvah itself?
Chasam Sofer (Orach Chaim 55, quoting from Peri Toar 28) writes that the blessing of Shehecheyahu applies to all mitzvos that the boy will now perform (as you suggest in the question), for he is now obligated in all mitzvos. Yet, the custom is to recite the berachah over the concrete performance of wearing tefillin on the day of the bar mitzvah, and to have intention (according to Chasam Sofer) for all mitzvos. See Yad Yitzchak I, no. 171.
The blessing is not made a month before, while the child has not yet become bar mitzvah, but only on the day of the bar mitzvah. Tevuos Shor (28) writes that the berachah was not instituted for somebody beneath bar mitzvah, adding that if a boy begins to wear tefillin before his bar mitzvah, he would not recite the blessing upon reaching bar mitzvah. See also Keren Le’David, Orach Chaim 57. However, Shulchan Melachim (p. 343) who writes that the blessing is recited upon reaching bar mitzvah, and this is understood in light of the above Chasam Sofer.
However, because some rule that a berachah is not recited over the first wearing of tefillin (see Shach, Yoreh De’ah 28:5, disputing the ruling of Rema 28:2; see also Taz, Orach Chaim 22:1, who rules that the blessing is made, and see also Maharsham (22) quoting from Rambam), it is customary to make the blessing on a new garment, and to have in mind the mitzvah of tefillin and other mitzvos.