I 100% agree with Barb. I don't know anyone whose kilt that fit their 12 year old body would be able to fit them as an adult (or even close, even with room to let things out).

Might I suggest this? Have the kilt made with a LITTLE room for expansion (2" hem) and have an extra 4.5 yards (double width) cloth woven up. In 10 years when he wants a new kilt, you'll only have to find a kiltmaker, not have a whole new bolt woven.