I have a couple of original patterns for A-line tunics, and they are not terribly full around. Think big shirt with a belt. The sz 6/8 one that I use most probably measures 36- 40" in circumference at the bottom. I can measure the pattern later if you like. this pattern: http://shop.originals-by-kay.com/product.sc?productId=1723&categoryId=53
In a toddler sized original I own, the body is a basic shirt shape flared out a little, cut to the length you want and straight across the bottom & buttoned down the front. Set in coat style or regular shirt (bishop) sleeve and belt loops for a self belt that buttons in front. this pattern: http://shop.originals-by-kay.com/product.sc?productId=1148&categoryId=53
I'm concerned that if you have too much fullness it will look like it is way too bulky around his waist.