"Aftersun" is intentionally ambiguous, which might frustrate audiences who prefer concrete answers. That Sophie's in possession of Calum's camera (and Turkish rug) all but prove he's passed away, as does the structure of the film, in which she's re-evaluating her memories and the videos from their trip. But, as the trope goes, if the audience never sees the body, the character isn't conclusively dead. It's possible (but highly unlikely) that Calum just stopped being a part of Sophie's life after their vacation.
The much more plausible explanation is that Calum continued to struggle with depression after Sophie boarded the plane home to her mom. From Calum's perspective, she zig-zags through the line and waves goodbye, which is the last thing he records. From the film's perspective, Calum walks down an airport hallway and into the rave. This could mean that he dies by suicide almost immediately after the events of the movie, or it could be an abstract final note meant to convey Calum's conflicted feelings rather than a definitive action.
If he did end his life, "Aftersun" doesn't seem interested in the details about how or when. It's about Calum warring between his role as a father and his reality as a person with severe mental health challenges. He's eager to share whatever wisdom he can with Sophie — how to defend oneself, for example — as if he knows he's on his way out. But he also makes promises as if he'll be around. Ultimately, it's up to the viewer. However, with loaded images like the Polaroid of Calum and Sophie that we never see develop, Wells seems to provide a tragic if still ambiguous resolution.
