Any damn time he feels like it. Seriously—a kilt is a perfectly acceptable garment for most any occasion except for swimming or skydiving.
The great thing about a kilt is that it is always appropriate—you just accessorize appropriate to the occasion.
If you are just going down to the pub with friends, a simple belt and plain sporran (to carry your keys and cash) is great.
For a formal event, the same kilt could be worn with a dress sporran, formal hose/socks and a variety of coatees/jackets/waistcoats appropriate to the occasion.
Just get one that fits—it should never hang past the top of your knee. That’s the usual mistake people make with kilts—they wear them too long.
Otherwise, kilt on!