1) I think there may be a few ways to explain why under some circumstances he has to keep 100 days and under different circumstances he has to keep 130.

For the moment I am going to explain, bs'd, according to Rabeinu Tam, cited by the Shitah Mekubetzes. He writes that it all depends on what he said first. If he said first, "I will be a Nazir when I have a son," and afterwards he said, "I will be a Nazir for 100 days," this means that he starts his 100 days now and when he has a son, he must stop his own 100 days immediately and start counting 30 for the Nezirus for his son. (This is because he said from the beginning that he will be a Nazir when he has a son, so the moment that happens he must put everything aside and start the Nezirus for his son.) Therefore, these 30 days count also as part of the 100 that he has already started counting, because he had to break off the first count in the middle, so when he goes back to the first count, he starts again from where he left off.

But if he said it the other way round -- first he said, "I will be a Nazir for 100 days," and afterwards he said, "I will be a Nazir when I have a son" -- he does not have to break up the 100 days. He completes the 100 days and afterwards starts the 30 for the Nezirus for his son. In that way, he ends up doing 130 days. In this case, he did not have to break off in the middle, so he keeps two complete counts.

2) The Rambam does not agree at all with the idea that one Nezirus is "swallowed up" within the other.

a) The Rambam (Hilchos Nezirus 4:4) writes, "If someone says, 'I will be a Nazir when I will have a son,' and 'I will be a Nazir for 100 days,' and he started his own Nezirus and then had a son, if 30 or more days remain from the 100 that he vowed by the time he had a son, he did not lose anything, because he stops his own Nezirus and starts counting his son's. He shaves, brings his Korbanos, and then completes the 30 or more days that remain from his Nezirus and shaves at the end of his Nezirus."

b) We learn that, according to the Rambam, he observes 130 days, not 100 days, even if he said he would be a Nazir for his son before he said he would be a Nazir for himself. The Rambam disagrees with Rabeinu Tam. The idea of Rabeinu Tam is stated in Tosfos above (13b, DH Hareini), who writes that the days of his son are "Muvla'in," swallowed up, among the 100 days, and count both for the Nezirus for the son and for his own Nezirus.

c) Tosfos Rebbi Akiva Eiger (on the Mishnayos at the end of the second chapter of Nazir) writes that according to the Rambam, we do not say at all that any days can be swallowed up. It seems that there is a fundamental dispute between Rabeinu Tam and the Rambam. The Rambam maintains that for each individual Nezirus that a person accepted on himself he has to observe independently all the days. Therefore, it must be a minimum of 130 in our Sugya. According to Rabeinu Tam, it can sometimes be only 100.

3) Why does the Beraisa (14a) say that he must keep 130 days?

There is a Beraisa cited by the Gemara (14a, 8 lines from the end of the Gemara) which states explicitly that he must keep 130 days. The glaring question is, how can Rabeinu Tam say that he must observe only 100 days if the Beraisa says 130?

a) The Beraisa reads like this: Somebody said, "I am a Nazir after 20 days and from now a Nazir for 100 days." The Halachah is that he counts 20 days and afterwards 30 days and afterwards 80 in order to complete his original Nezirus. This is because he said he is a Nazir from now, so he must start his Nezirus immediately. After 20 days, he starts the Nezirus that he declared he was going to start at that point, which means he counts another 30 and has now kept 50. Since he has only kept 20 from the first Nezirus, this means he still has 80 to make up to 100 from the first Nezirus, so 20 + 30 + 80 = 130.

b) This seems to clash head on with what Rabeinu Tam said, as cited by the Shitah Mekubetzes, that we saw in my earlier answer. Rabeinu Tam said that if he breaks off in the middle of one Nezirus in order to observe another Nezirus, then the days of the second Nezirus also count for the first. Why, then, in the Beraisa on 14a do we not say that since he broke off his first 20 days in order to keep the 30 days that he mentioned first, it follows that this is a broken-off Nezirus, and the middle 30 days can be added to the first 20 days and he will only need 50 more days, not 80 more days?

c) This question is asked by Tosfos (13b, DH O Dilma). Tosfos writes (seven lines from the beginning of Tosfos) that the Mishnah on 15a states that if the son is born within 70 days, he has not lost anything and the days of his son's Nezirus help to make up 100 days. The next word in the old text of Tosfos is "v'Hachi," but Hagahos Maharav Rensburg (#9) corrects this to read "v'Heichi," which represents a question. This means that Tosfos is asking: Why does the Beraisa on 14a say that after keeping the 30 days he must keep another 80? Why do we not say that the 30 days of the second Nezirus are "swallowed up" in the 80 days?

d) Tosfos answers that the potential father in the Mishnah on 15a had different intentions than what we notice at first sight. His intention from the beginning was that if he will have a son before 70 days, then he will never be a Nazir because of his son. If he will have a son before 70 days, then he intends from the beginning that his son's 30 days will be swallowed up in his own 100 days. The only scenario where he intended that he would be a Nazir because of his son is where the son was born after 70 days.

e) Tosfos writes that the Beraisa on 14a is different from the Mishnah on 15a. On 14a, the person accepted Nezirus after 20 days and he did not intend that those 20 should be included in the 100 that he accepted now. If he would have intended the 20 to be included in the 100, he should have said, "I will be a Nazir in 20 days' time and I will start now 80, so it will be a total of 100." The fact that he did not say that proves that he wants now to accept 100 days in addition to the 30.

f) The difference between 14a and 15a is that in 15a he did not know when the son would be born and he only wanted Nezirus for his son if he would be born after more than 70 days from now. In 14a he knew exactly what he was getting himself into, so we see that he accepted 130 days right from the beginning.

Dovid Bloom