It is said by an adult man to a young man.
おれっちのみたこと あの子も かなーりの盗賊のうでを持っていそうだったぜ。 まあ あの子に負けねぇおとこにはならんとな。
In my understanding, the word 「ならんとな」 consists of 「ならない」, which means 'not become', and particle 「とな」to emphasize his statement.
However, when I tried to translate the later sentence, it turned out like this.
Well, you should not become a man who does not lose to that kid.
, which I think it looks opposite to my sense.