I'm in the process of making my 6 year old brother a new coat (he's outgrown two coats in the three years we've been reenacting, the little stinker!

) out of some oatmeal colored wool I got at Joann's recently. I'm using a man's shell jacket pattern and sizing the basic shapes down to fit my brother. It's going to be slightly different - replacing the military stand up collar with a fold-down collar - and maybe a breast pocket. It'll probably have about 4 buttons.
So, my current question is - should a little boys coat have a three/four piece back (like the shell jacket pattern) or a one piece back?
Are there any other differences I should know about between mens coats and boys coats?
thanks!! -A.