A first aid skill check can treat a single wound and restore 1d3 hit points (1d3+3 on a crit). That can only happen within an hour of the wound being sustained. If he does not rise above zero HP by midnight, he will die.
Arne has sustained two wounds in the past hour, so you have two chances to restore hit points. Only one person can treat a specific wound, and each success restores 1d3 HP.
Left alone, he will die by midnight. But even if he gets first aid, he may still die.
Even if he survives, he has sustained a major wound and will require chirurgery. Even if he survives the night, he is out of action this campaign season.