I was just at my Wal-Mart yesterday and they didn't have ANY of these jackets, because I too am looking for one for Stedman, although I have no idea what size he would need. I will keep checking my Wal-Mart and will most definitely let you know if I find one in a size medium for your little guy!
After all, you are the one who found the little Spooky Spider Dress for me, LOL