How did Chris react when he sees boom! Sonic and the moon others?
I guess he had that one initial reaction of when someone just spaces out for a couple seconds in disbelief, as if he was seeing a ghost, before quickly coming back to reality.(Whatever you call it, beyond just using the “TRIGGERED” meme.) After that he would probably genuinely question to himself if’s the Sonic; as in the one he knew from his universe. Same goes for Boom!Tails, Boom!Knuckles, and Boom!Knuckles, but it would be the worst with Boom!Sonic.
Needless to say, Chris would be having a very hard trying not to project the Sonic and his friends that he used to know onto their Boom counterparts, and not treat them as such. To the point of slipping up at points. Like already calling Boom!Sonic by, well, Sonic, before he could even have a chance to introduce himself- let alone in a tone that would imply that he somehow already personally knows him. But at least he could brush that one off by saying that Sonic is popular on the island and all, so of course he already knew his name. But then there’s the instances like asking questions, or mentioning things that Chris knew from his old friends, but would just come off as completely random or out from the left field from the Boom incarnations.
Not sure what you mean by the “moon others”, though.