A lot of people recommended delving into lore channels but I want to recommend an even easier approach, watching the gameplay or "movie" version of the Destiny 1 and 2 expansions or at least videos that have a compilation of the cutscenes, this will help give a very basic layout of what has happened in the game. I remember trying to watch one of the long lore videos from Byf and he started with the events that took place BEFORE the base game it was hard to follow imo. Good news, the stories in the destiny franchise are really short compared to other games.
The order of release is
Destiny 1 (base game)
The Dark Below
House of Wolves
The Taken King
Rise of Iron
If your son still has Destiny 1 I recommend playing it if you have the time.
Destiny 2: The Red War Campaign (base game)
Warmind
Curse of Osiris
Forsaken
Shadowkeep
Beyond Light
Witch Queen
Destiny 2 has seasons sandwiched in between each big expansion it contains added story.