"He is a kind father."
Translation:Ille est pater benignus.
Adjectives must agree with the noun they modify in gender, number, and case. pater here is masculine, singular, and nominative. benignus is the masculine singular nominative form. benignum could be masculine accusative singular or neuter nominative/accusative/vocative singular.
No, the accusative should absolutely not be used.
The verb esse does not take a direct object, there is no action being done. It takes a predicate nominative which describes the subject.
Ille - 'He' is the subject
pater benignus - 'a kind father' is the predicate nominative.
It's not a different person, thing, etc. est just links them together saying that the predicate nominative is apart of the subject.