Fallout – When does X6-88 become available as a companion

Followers can not die. Also noted in this article about companions that can't die.

Good news: no matter what transpires out there in the wasteland, he cannot die.

In fact, none of the companions that may accompany you on your quest across post-apocalyptic Boston can die. Speaking with IGN, Bethesda's Todd Howard discussed the now-immortal companions, a decision the development team made to better serve the player.

Since they can't die, doubt shooting them will bother them much.

There's a workaround in the wiki: (WARNING: Wiki is full of spoilers; I've copied the section below and censored spoilers)

Bugs

• There is a known bug where after the quest to reclaim the Castle it becomes impossible to get him as a companion. Wait until dealing with [SPOILER] and visit the [Brotherhood of Steel] before completing the quest, so this bug does not affect the Sole Survivor.

  • This can be fixed on PC by opening the console, selecting Preston Garvey and inputting: removefac 00075d56

I've read somewhere that all this faction code does is block companion capabilities, which makes little sense as the last line of dialogue for him is that he'll always have my back.

Anyway, you have two options (unless you play on PS4/Xbox One, in which case #1 is your only option):

1. If possible, reload a save right before taking the castle and continue with the main quest, until you "clear" the location Fort Hagen; or
2. Open the console, click Preston and type the command.

I used the console command and I didn't see any side-effects, even a little further in the main quest. I still highly recommend doing a full save (manual one, not auto/quick) before doing this. You can also restart the game afterwards to ensure the game state is stable. (Though that's really not needed and mostly voodoo on my part)