We are postponing the Seudas Mitzva 2 days due to logistical / technical issues to make it easier for guests from out of town.
I understand from the Yam shel shlomo that we are fulfilling the seudas Mitza with the Derosho and celebrations. Is there a requirement do mark the occasion on the actual birthday (Bo Bayom) in addition and if yes what are the requirements (Minyan, meaty meal, etc.)?
Mazal Tov! Your son should grow up to be a big talmid chacham, and enjoy doing the mitzvos.
Essentially the real Bar Mitzva seuda is on the day of his actual Bar Mitzva. You don’t need a minyan, and there is no specific obligation to have a meaty meal, but it would be proper for it to be a meal that honors the occasion. We make a nice meal to celebrate the mitzvah of Bris or Pidyon Haben, a Bar Mitzva Seuda essentially is a meal that celebrate all of the mitzvos!