。
It wasn't until dawn that he finally fell asleep.
Literal
Dawn [to-に]-becoming-[て] for-the-first-time he [topic-は] fell-asleep.
~になって初めて ('not until X, only after X') is a fixed pattern emphasizing that something only happened after a certain point — literally 'becoming X, for the first time.' 寝つく ('fall asleep') is a compound verb specifically for the act of drifting into sleep, distinct from 寝る ('go to bed, sleep'). The sentence implies insomnia up to that point.