How to stop my players from dying to Venomfang?

One of the PC's in my game tends to rush in with his sword first, and think later. He charged nothic alone, on lvl 2, on low HP. The only reason he survived is because nothic isn't bloodthirsty and didn't chase to not risk the fight with the rest of the party.
I think this guy might be problematic when they finally reach Venomfang. I don't think dragon would let him escape, and breath weapon might knock him out in one round. That's why I need some ways to do a deus ex machina moment and save his stupid ass (i don't want my players to die if they don't have access to revivify).
Since I want to go with Storm King's Thunder after this, I though about bringing in Klauth in this moment. He sort of helps players later in STK so it's good to introduce him sooner. He's also known for killing other dragons, so there's a good reason for him to be there. He's also a bit random in his actions, sometimes even helpful to those he doesn't consider a threat. And if for some stupid reason, my idiot player would try to attack him, I have legendary action to just tail swipe him away like a fly.
What do you think about this? Isn't it too random? Should I figure out something less grand?