To put this into context, last year DS was a ghost - this was achieved by taking a white sheet and cutting a hole for his head! As you can see 'arts and crafts' are not my forte 
So this year he wants to be a clone soldier - and thats easy because you can buy the costume (basically a white jumpsuit with markings on it and fake boot covers on the bottom of the legs). Except even the smallest size is too big by about 2 inches. So bearing in mind my complete lack of skill in this area, what is the best/easiest way to make it smaller and ideally in a way that can be removed as he gets bigger?
Options I've considered
i) elastic round the waist - to begin with he'll be a baggy clone soldier but it should hold it up enough and the elastic will stretch as he grows
ii) fold and tuck about 2 inches around the middle and stitch it in place, then unpick as he grows
iii) give up - cut off the ends of the arms and the ends of legs (plus fake boots) and buy a new one next year?
Any advice gratefully received 