If a boy becomes Bar Mitzva during Sefira, may he take a haircut for the Simcha - similar to a Bris where the father is allowed to take a haircut in Sefira?
Nit'ei Gavriel (Pesach, vol. 3, 50:11) cites authorities on both sides of the issue:
The bar mitzvah boy and his father are indeed allowed to take haircuts (Mekor Chaim);
They are not (Rivevos Ephraim);
The boy can have his hair cut before his actual bar mitzvah date, when he's still a minor and not fully obligated (Divrei Shalom);
If his hair is really long (which would also interfere with his putting on tefillin), then everyone would agree that his hair can be cut - although it should be done before his actual birthday (Nit'ei Gavriel's own opinion).
Practically, as usual, CYLOR.
No. The simcha of the barmitzvah is our joy in the boy becoming obligated to keep mitzvos and the Jewish way of life.
When will he have his haircut?
If he takes it after his birthday, we would introduce the barmitzvah boy to mitzvos by making him go against our practice which is surely not the right thing to do.
if he takes it before his birthday, what sort of chinuch - education is that for his adult life?