Q. Can a Bar Mitzva boy who is to be Bar Mitzva on Sunday, read the maftir and the Haftorah on the immediate Shabbos before when three Sefer Torahs are read?
A. Poskim maintain that one can call for maftir a kotton or minor, not yet bar mitzva, since the maftir portion is not part of the count of the seven required for the reading of the Torah. (Mishna Berura 282: 12). There are exemptions to the above, such as Parshas Zachor or Parah, when no minors are called.
When two or more Sefarim are read, such as on the Four Parshios, Ribosh (326) and others maintain that since maftir is not repeated as it is usually done on other occasions, but just read once in the second or third Sefer Torah, it is not correct to have a minor be the one being singly called to read it. (Eliahu Rabba, Tashbatz 1: 171, Ginas Veradim 1: 34).
However, some Poskim are lenient and follow the Remah’s (282: 2) opinion and permit (Shulchan Aruch Horav 282: 16, Chaye Adam 31: 39, Kitzur Shulachan Aruch 79: 9 and others. See also Mishna Berura ibid. 23 and Biur Halocho).
Horav Shlomo miller’s Shlit’a opinion is that in need one can be lenient. Additionally, the maftir sefer may be read twice, once by the baal koreh, and then repeated by the minor. (The Rov pointed to Sefer Hamanhig 150).
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a