The reason why he looks like an older Kurt Russell when he meets his adult son is also explained in the movie, but can’t be revealed here just yet!
(it’s worth noting that he didn’t do this to dupe Meredith, as it was established in the first Guardians that she knew that Ego was an alien).
And the timeline certainly matches.
As seen at the start of the first Guardians of the Galaxy, Peter Quill was eight years old when he was abducted in the year 1988 — which means that Ego and Peter’s mom, Meredith, were together circa 1980.