He is afraid. He is totally alone. He is 3 million light years from home.
Why it is hard sci-fi
Soft. The biology is benign and the science is window dressing. Where the film does work is in showing first contact through the eyes of a child, with adults as a slow-moving threat, and treating the alien as a botanist on assignment rather than a metaphor. The covert biocontainment operation that takes over the house in the third act is the only sequence that engages with real procedure.
Science inside it
Biocontainment, suit-up protocols, decontamination tents. The film also touches on cross-species heart-rate synchronisation as the link between Elliott and ET, which is closer to magic than physiology, and the film knows it.