I told him and now he knows.
I told him and now he knew.
If I have now in the sentence. Does it mean that the following verb needs to be in a present tense? Or it is not necessary?
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Both are. Since "now" can refer to a past present by conveying the verb it modifies as newly happening in that "now" moment in the past, both ways of saying it are grammatical, but they are used in different situations to convey different meanings.
So, you have to decide what it is you're trying to say to know whether to use "now he knew" or "now he knows," the former conveying he came to know it in that past moment, perhaps to then go on to say what he did next in the past as a result of knowing that, and the latter conveying that he knows it in this present moment, perhaps to then go on to say what he should do now or will do now in this present present or what obligations or responsibilities he now has in this present present because of having that knowledge.