QUESTION: The Mishnah states that when a person says, "I will become a Nazir when I have a son, and I am a Nazir [now]," he begins to observe his own Nezirus (the second Nezirus he accepted in his statement) right away, and if his son is born after twenty days he stops counting his own Nezirus and counts thirty days for his son's Nezirus (the first Nezirus he accepted in his statement). Afterwards he counts another ten days to complete his own Nezirus.
The Rishonim ask why does it suffice to count the last ten days for his own Nezirus? If he shaves after he observes his son's Nezirus, he has only a ten-day growth of hair on his head. Instead of observing ten days, he should observe thirty full days of Nezirus in order to grow his hair long again!
(a) TOSFOS explains that at the conclusion of the Nezirus for his son, he simply does not shave or bring the Korbanos of his Nezirus. Instead, he shaves ten days later (after he finishes his own Nezirus), and at that time he brings both sets of Korbanos -- for his son's Nezirus and for his own Nezirus. Although he is required to put all of his hair into the fire underneath the pot in which his Korban Shelamim cooks, and in this case there are two Korbenos Shelamim, he may put both Korbanos in a single pot and then put the hair underneath that pot.
Why, though, may the Nazir delay the shaving and the Korbanos for the Nezirus of his son? The Torah commands a person not to delay bringing his Korbanos (Bal Te'acher). The answer is that the Gemara in Rosh Hashanah (6a) teaches that the prohibition of Bal Te'acher applies only when a person delays bringing his Korban for three festivals, and one transgresses the Mitzvas Aseh against delaying his Korban only when he delays it past the upcoming festival. If he delays his Korban for just a few days, but he brings it before the upcoming festival arrives, he does nothing wrong.
(b) The ROSH suggests that perhaps the Nazir does shave when he completes the Nezirus for his son. When the Mishnah says that he observes only ten more days for his own Nezirus, it means that he counts ten days of his Nezirus and then another twenty days of "Gidul Se'ar," of growing his hair. Those days are not considered part of his actual Nezirus. A Halachic difference between days of "Gidul Se'ar" and days of Nezirus applies if he becomes Tamei during the days of "Gidul Se'ar." Becoming Tamei does not annul the days of Nezirus he already counted. Becoming Tamei annuls the days he observed only when he becomes Tamei during the days of Nezirus, as Rav says (14b). (Shmuel argues and says that Tum'ah annuls the days of his Nezirus even during the days of "Gidul Se'ar." The Rosh suggests other practical differences which apply even according to Shmuel.)
Why does the Rosh insist that the Nazir shaves his hair at the conclusion of the Nezirus for his son? If, by shaving his hair, he will need to observe another twenty days of growing his hair, why does he not just delay his Korbanos until his own Nezirus is over (in ten more days), as Tosfos explains?
The Rosh apparently understands that if he does not shave after the Nezirus for his son, he cannot return to count his own Nezirus. There must be an act of shaving and bringing Korbanos which denotes the conclusion of one Nezirus and which separates his son's Nezirus from his own. Hence, if he would delay his haircut for another ten days, he would still need to observe another thirty days for his own Nezirus after he cuts his hair for his son's Nezirus.
Perhaps the source for the opinion of the Rosh is the Mishnah earlier (8a). The Mishnah states that a person who accepts upon himself to become a Nazir as many times "as there are hairs on his head" becomes a Nazir forever. He constantly renews his Nezirus and shaves every thirty days. The ROSH there questions whether the Nazir must shave every thirty days, or whether he may delay shaving until the festival (and bring only one set of Korbanos then), since he does not transgress Bal Te'acher until the festival passes. The Rosh concludes that it is logical to assume that he must conclude each Nezirus, and bring Korbanos for each one, after every thirty days. Since he accepted upon himself to observe an unlimited number of sets of Nezirus, he is obligated to fit as many Nezirus periods into his lifespan as he can in order to fulfill his oath. This seems to be the intention of the Mishnah which states that "he shaves every thirty days" and not that he shaves at every festival. If, however, a second Nezirus can take effect before he shaves for the first Nezirus, why should he have to shave and bring Korbanos to the Beis ha'Mikdash every thirty days? He should wait until the festival to shave and bring Korbanos, and the Nezirus will automatically re-start every thirty days (even though he did not yet bring Korbanos), and on the festival he will bring a set of Korbanos for every thirty days that passed. Why does the Mishnah require that he shave every thirty days, instead of requiring that he start a new Nezirus every thirty days, or that he bring at the time of the festival a set of Korbanos for every thirty days of Nezirus he observed? It must be that the Mishnah maintains that shaving is necessary in order to end one Nezirus and begin the next. This is why the Rosh writes that he must shave immediately after he observes his son's Nezirus before he observes his own Nezirus.
A more obvious source for the Rosh's view may be the opinion of the Chachamim (18b) who rule that a Nazir Tamei may not begin his Nezirus Taharah until after he brings his Korbanos for the Tum'ah. Although Rebbi Eliezer disagrees and says that he starts the Nezirus Taharah before he brings the Korbanos for his Tum'ah, the Halachah follows the view of the Chachamim.