Should versus must

[Rustamsher]

He is studying hard now, so he ... pass the exam.

(a) must
(b) should


I think both are grammatically correct and appropriate for the context (although I favour the "should" in this case). Am I right, teacher?

If not, which is the more idiomatic?

Thanks.

[Alan]

Both grammatical, but only 'should' is sensible (i.e. indicating simply the speaker's expectation rather than some inarguable logical inference).