I don't think you are going to find an off the rack kilt that will fit your son and based on the measurements a hemmed adult kilt probably wouldn't fit that well.

A possibility that might work, until it is worth getting a custom kilt for him, is a Buzz Kidder kilt. Myself and others on the forum carry them. In your situation they have two advantages, first the length can be ordered to suit your needs so there is no hemming after the fact. Second, since the fell is on the short side, compared to other kilts, it should work very well in this situation proportion wise. Depending on the seller you should find a price less than $125. Based on your son's size requirements I don't think you will do much better. PM me if you need more info.