Check basic commercial patterns like Simplicity, McCalls, etc. for your smock pattern. JoAnn's alternates having one brand on sale for at least a week a month, so never pay the ridiculous full price!
For your precious son's costume, I agree on sweats as base. Felt dog ears and a blackened nose, with a few felt spots and a tail; Cat ears, whiskers, and tail; a big "S" and a red cape...