Nah, it's issue #10 with JP's origin story. I've got a couple of the relevant scans here.
I guess I should add that I don't think it's a forgone conclusion that Mac did intend to out Northstar as a mutant or as homosexual. I'm pretty sure he wouldn't have, given what a boyscout he could be. My thought is that he was working with the leverage he had on hand and was hoping that it would be a stick then carrot approach, with Northstar coming around to his way of thinking once Mac got an opening in by unsettling him. Maybe Mac was even thinking there would be some gratitude involved for letting him know he was a Beaubier. But given that JP gave zero fucks about his actual parentage and was utterly unintimidated by Dept H, Mac's digging expedition, or anything else, it was exactly the wrong approach to take.
He was kind of a jerk not to give Jeanne-Marie warning that her brother was coming, though.
And Northstar is supposed to be in at least the next arc of Amazing, yes.