I actually don’t think it’s as outrageous as some people make it out to be. McKenna is ultimately a surname, and surnames have always moved between genders more freely than given names. Historically, it also isn’t a name that was exclusively feminine, it only became heavily associated with girls relatively recently, like someone else pointed out McKenna Grace is a fairly popular actress right now.
That said, perception matters just as much as history. At least in the US (and Australia too), McKenna is probably going to be read as a girls’ name. A boy named McKenna would almost certainly be mistaken for a girl on paper, and he’d probably be asked about his name fairly often.
I do think the nickname Mac or Kennie helps a lot. Mac is undeniably masculine and gives him an easy everyday option if he ever wants it. I also think it’s worth remembering that names like Ashley, Lindsay, Kelly, and Mackenzie all spent time as male names before shifting, so today’s perception isn’t necessarily the same as historical usage.
It’s interesting that people rarely question boys’ names being used on girls, but the reverse is treated as off-limits. If we’re happy to embrace gender-neutral naming, it shouldn’t only work in one direction.
If McKenna has deep family meaning, I think that’s an even stronger argument for using it. To me, a meaningful surname name on a boy feels much more intentional than simply choosing it because it’s trendy. I honestly think it works.