Some of my friends and I are actually planning this! Our thoughts were to get a black or brown long sleeve shirt or sweater to wear. Then get two extra shirts or or sweaters and cut the arms off them and stuff them. Sew them to the sides of the main shirt that your son will be wearing and tie each of them to the arm above to give the illusion of have six arms. You can also get a head band and tie or glue pipe cleaners to the band and attache foam balls to the top for the antenna things. Hope this gives you some kind of idea.