Hi, Muffinman, and hearty congratulations to your son on making Eagle Scout.
During the two years I was a Scout, South Africa went from being a British dominion to a republic outside the Commonwealth, so we also saw a transition from Queen’s Scouts to Springbok Scouts.
My patrol leader was a Queen’s Scout – one of the last in South Africa.
The BSA uniform rules do strike me as odd (okay, I know this is a red-button issue, but I don’t plan on making a major issue of it).
In my day all Scouts wore short pants (military-style khaki). Rovers could be seen in trousers, and some Scoutmasters wore them. But our Scoutmaster wore shorts, as did the Scoutmaster next door (two troops shared a hall, which was divided by a partition).
But the trousers were always khaki, even if (like the neighour troop) the shirt they wore was green. Most of the Scouts in Cape Town wore khaki shirts.
One troop (from a Presbyterian church) had a tartan element in their scarves, but I cannot recall whether the scarves themselves were tartan, or if the scarves had a patch of tartan on the triangle.
While I do not recall seeing a kilted Scout in the flesh, we were well aware that Scottish Scouts wore kilts.
I have noticed trends in Scouting in various countries towards putting the boys (and nowadays the girls, too) in trousers. But grey trousers strike me as being extremely odd with the Scout uniform.
Perhaps that is because I am used to having worn (in the distant past), and seeing boys wearing, grey trousers as part of school uniform, by contrast with Scout uniform.
The only difference (in this respect) between the schools I knew in Cape Town and those in the Eastern Cape was that in Cape Town, boys in Std 6 and 7 (Grade 8 and 9) wore shorts, while only the senior boys wore long pants, whereas in the Eastern Cape younger boys were also expected to wear longs.
But to this day, grey shorts are the uniform in primary schools (Grade 1 to 7), not trousers.
The point of this ramble is that, at least in South Africa, grey pants are not Scout uniform.
As for kilts, I don’t even know Scout uniform rules in South Africa any more, but I would recommend wearing a kilt for hiking.
Someone (on this thread or another) warned about ticks and other pests that might infest one if you walked through brush, but it is worth mentioning that many of these little pests prefer to infest clothing like trousers, and are less fond of bare legs (and a kilted man’s legs are much barer than a man in trousers).

Obviously Muffinman’s son would need permission to wear a kilt to his court of honour, but not simply to possess one. I would say go for it – get the kilt, young man!
Whether that kilt should be McLaren is also something I will not pronounce on, but possibly Muffinman junior does have his Wood Badge already.
Regards,
Mike