"MUSTN'T" or "CAN'T"? (in expressing improbability)

He's a nice boy. It can't be him stealing the pencil! - CORRECT

He's a nice boy. It musn't be him stealing the pencil! - WRONG


*Must is NOT often used to express certainty in NEGATIVE clauses.
It is used to express the conclusion that something is certain or highly probable.

E.g.,
He's a bad boy. It must be him stealing the pencil! - CORRECT!