There is a company based out of China that makes all sorts of costumes and sells them thru ebay. They are cutom tailored. You send the measurements and they make it for you. I just ordered a costume from them. Their reviews are excellent and the costumes appear to be very high quality.
Here is a link to an Evil Kinevil costume that they sell
Go to a local costume/theatrical shop if you don't mind spending a little money. They can usually make stuff from scratch that they'll actually tailor to your son.
Buy or take whatever costume you can find to a tailor and have it custom fitted. I've bought several cheerleader uniforms and had them cropped to my liking.