I bought these in a Husky and they have a 3" stretch of elastic in the back.  That makes them super comfy and gives my little dude some room to grow so they will last longer.  I have also noticed that after a few washings this style of the Dickie shorts tends to get a bit softer than the adjustable waist or the extra pocket styles do.  My son has had all three and these are by far my favorite.

I used to get these from the school uniform supply place but the prices here are better, and for the Husky sizes, there is an even greater discount.