Even if he had spelled your correctly, he still would have been wrong. The proper usage would have been, "You are so stupid." So, what he should have said in this case is "You're so stupid." Your is a word to show posession, as in "your stupid idea" or "your weapon won't work for that." You're should be used in any case where "you are" is the appropriate usage.
English grammer 101 class dismissed. :D
Sorry, I had the sudden urge to expound upon a subject that no one is really going to pay that much mind. :)
