How do I save Matt?

My understanding is that at a certain point in the story, Matt will die unless he has the flare gun to defend himself. A guidance totem told me to give it to him, which I did, and he immediately fired the flare into the air, wasting it. Reading online indicates he doesn't always fire it.

What choice did I make earlier that caused him to fire it? Is there any other way to save him without it? Is the guidance totem a dirty liar that lies?

Trying not to give away too many spoilers here, but read at your own risk anyway:

It is still possible to save Matt even if he wastes the flare gun. In this case, you have to choose to immediately jump instead of trying to rescue Emily when she is hanging from the toppled tower.

However, if you do not want Matt to waste the flare gun, you must disagree with Emily when she says to go to the tower. In this case, Matt will not shoot the flare gun, and you can attempt to rescue Emily.

Also you must not attack the deer.

More information here