Nazir 2:9

If a person says, “I am a nazir, and a nazir when I have a son,” then he starts his nezirus period and a son is born, he must first complete the nezirus he undertook for himself and then begin the nezirus he obligated himself for his son. If he says, “I am a nazir when I have a son and a nazir,” he starts his own nezirus period. If a son is born, he suspends his own nezirus period, observes the one for his son, then resumes and completes his own.

Nazir 2:10

If one says, “I am a nazir when I have a son and a nazir for 100 days” and a son is born before day 70, he loses nothing. After day 70, the subsequent days don’t count because there is no tiglachas (shaving with related sacrifices) for fewer than 30 days (i.e., when he resumes his count, he must observe at least 30 days no matter how many of the 100 days he observed before he paused that nezirus to observe the one for his son).