、。
In short, he was the careless one.
Literal
Essentially, he [subject-が] careless was-[explanatory-のだ].
が here marks 彼 as the identificationally-focused subject — 'it was specifically he who was careless,' distinguishing him from anyone else. 不注意 ('carelessness') is built with the Sino-Japanese negative prefix 不~ on 注意 ('attention'). The な-form 不注意な is the modifier; here it's used predicatively with だ.