Difference between "dawn" and "realize"

The usual meaning of "dawn" is "sunrise". It is used in a poetic way here to describe that after a long period of darkness, where the person just did not realize something, he finally came to the right conclusion.

The word "realize" itself makes no implication about how much time it took the person to come to this conclusion.

The verbs dawn and realize work in opposite directions. In 1, the idea is the subject and he is the object. In 3, he is the subject and the idea is the object.

So, they are not synonyms.

Dawn is not a synonym for realize; however, the verbal phrase dawn on could be considered synonymous.

The trick is finding a dictionary that recognizes (and lists) dawn on as an entity in its own right.

From Macmillan:

dawn on someone if something dawns on you, you realize it for the first time: It was several months before the truth finally dawned on me.

From Wiktionary:

dawn on (idiomatic, of an idea) to occur to somebody; to be realized: It finally dawned on him that he could automate the process instead of doing it by hand each time.

Both reference realize in their definitions.