Hi Maddy,
In the first one he might be too young to grow a moustache, the second and third... well.
He certainly doesn't look like a volunteer to me (in that to me it looks like a pukka RA uniform, but I'm not an expert), so the mystery is on.
You mentioned in your email to me that the headwear might be datable - I can't tell for sure whether it's a glengarry or a forage cap. If it is a forage cap, it dates the picture to the 1890s or early 20th century. It looks to me more like a forage cap, but as I say, I can't be sure. Over to the uniform experts...
I suppose the big questions are, when for sure did moustaches become compulsory? Were there any exceptions allowed - for instance in Army Orders or specific corps regulations (like I suspect existed for pioneers)?
EDIT - on the first of these - I have just found out that in the 1868 regs, no 358, moustaches were to be worn, so it is likely that this was the case throughout the period. Doesn't help solve the mystery though
Chris