As I understand, there is no mitzvah to make a se'udah for a Bar Mitzvah, unlike what occurs for a Brit, Pidyon Haben, Wedding or Siyum Masechta. Assuming that there is no siyum done, can a person serve meat during a Bar Mitzvah party made during the 9 Days?
Rabbi Daniel Neustat quotes the Yad Efrayim 551:31 and Divrei Yatziv 2:238 as permitting meat for a Seudas Bar Mitzvah on the day of the Bar Mitvzah.
However, for this and all Seudas Mitzvah dispensations, if it is during the week that Tisha B'av falls out, only a minyan plus close relatives may partake of the meat and wine (Mishnah Berurah 551:77). Sha'ar ha-Tziyun 84 is more stringent allows only a minyan including relatives to partake.
Since it is not a seudas mitzvah, the minhag would not be overridden and the prohibition would still apply.