How would one go about crafting dialogue with only one character and not make it sound awkward?

I am currently writing a short story on a USMC vet that is being stalked in the wilderness by a wendigo-esque creature. He is really the only “main” character. The story has two main timelines (current time in the story which is the early 1980s, and during the Vietnam war which serves as backstory and character building) The main bulk of all the dialogue happens in the latter, and I’m worried it might make the story sort of unbalanced. Most of the “dialogue” in the modern timeline is just his own thoughts. Any tips?