It kind of looks like a men's button-up shirt, minus the collar, roll up the sleeves, and add tails. Since the tails aren't part of how the jacket will fit, you can literally just draw them until they look right to you. I would also suggest trimming the front a little bit, since a men's button-up is meant to close all the way and even overlap a little. Make sure to cut out the pattern size that is a size too big, so it has the same sense of volume that simon's has. Something like this would be perfect, since it's flat across the bottom:
http://www.simplicity.com/p-2184-boys-a ... pants.aspx