SHOW, DON'T TELL
Avoid "prophesies" or "maid and butler dialog" about those almost-forgotten immortals that would telegraph what is coming. This is "telling, not showing". It should be a surprise to both the characters and the reader if it is meant to have any narrative impact.
The first time he is shown being immortal should feel like both an unexpected plot twist for the reader and a polarizing event for some of the other characters. Some will be shocked, some will suspect he is unusual, and some will dismiss it altogether.
The early incident can be left ambiguous, but now the characters have a reason to discuss immortals. Various characters will personify the various "opinions". Early believers will be seen as religious kooks while skeptics are presented as rational. At this point it is less about whether he is actually immortal because the reader hears all the worldbuilding lore and differing opinions: immortals don't exist, immortals maybe existed once but certainly not now, even if there are immortals they wouldn't look like that guy, that guy is a charlatan and this whole immortal thing is some kind of hoax, and behold the new immortal.
There is more than one "truth". With a range of reactions the reader learns about the range of the world's beliefs (richer worldbuilding) and the characters begin to differentiate themselves by how they react to the unexpected. Some are flexible and open-minded, some want to believe without proof, and some are suspicious or dismissive. Later when evidence of immortality happens again, these same characters will be given a second chance to assess the information. Some will change and some will be dogmatic. This tells us about what kind of people they are, while simultaneously representing the "rational" and "irrational" possibilities.
An author should not tell the reader what is "true" ahead of the story. Instead, show the events and allow the characters to react according to their own "truths". Giving an MC magic awesome superpowers is gee-whiz cool, but having a group of witnesses and ultimately an entire society become divided over the potential/threat/divinity of an "other" drives conflict, character, and storytelling.
Too much foreshadowing sucks life out of the story
Here's a quick example using the basic plot "Man in a Hole" (literally):
TELL (too much foreshadowing)
There once was a legend of a hole in the ground. Many people foretold the hole's curse, and said
"Someday, someone will fall in that hole!"
Joe is getting married today. Joe falls in the hole.
There is no conflict or surprise – someone was going to fall in the hole eventually. We assumed it would be the main character. This story is about a hole that fulfills its destiny. Joe is just the lucky guy that finally falls in.
SHOW (without any foreshadowing)
If Joe is late for his own wedding Susan is going to kill him. Joe
takes a shortcut and falls in a hole! He calls for help but no one
hears and Joe eventually crawls out. Late for his wedding and covered in dirt, he finds
Susan in the arms of his best man! Joe explains that he fell in a
hole, but Susan doesn't believe him. "Oh, Joe! There's no such thing
This story appears to be about Joe's mishaps, not a magic hole. There is character conflict that needs to be resolved. The hole is not more important than Joe or Susan or the wedding. We have a good idea where this story is going, but also some people don't believe in holes – wait, what?
Joe describes the hole. His inlaws think he's crazy. Susan is
humiliated but he is sticking to this ridiculous story. Grandma
mentions there's a witch at the edge of town who knows about holes, as
she gulps down the wine. They'd paid the catering in advance, wedding or not.
Joe finds the witch at the edge of town and for $25 she recalls an old fable
of the hole – which she dismisses as just a superstition. "Anyway, it's not
the hole that you have to worry about, it's the curse!"
If you give clues in advance, there is nothing to discover. You won't be able to raise the stakes because you've already let the reader know your endgame. Readers aren't stupid, they will recognize Chekov's gun when they see the finger point at it.
Allow the reader to discover things as the characters do. This creates empathy. It sounds like in your story the evidence will build over time anyway, so let it. Allow your characters to fall in the hole oblivious. Your characters should be lost in their lives so they don't see the hole coming. Once they climb out, they don't realize the full implications.
In the OP, immortality isn't important at all to the characters – until it is. It should be the same with the reader. They will follow what is important to the characters so keep your characters busy and don't give away the interesting parts until it fits dramatically with how the characters experience it.
The MC and his friends observe some odd things which don't seem important, so it should not appear as immortality or fate. Readers will get the idea when the story begins to revolve around it. Keep your readers in the present and allow them to discover the plot through the characters.