May a boy be the baal korah for layning on Thursday before his actual bar mitzvah 13th birthday on Friday?
Mazal Tov, may he grow up to be a big talmid chacham, and a yoreh shomayim.
Regarding your question, a boy before his bar mitzvah cannot be motzei the tzibbur with anything, therefore he cannot layn, even on Thursday, if it is before the day of his bar mitzvah. The exception to this is maftir, because it was already read and the tzibbur was already yotza with the bal koreh.
MG:A 162-6, M:B 162-13.