the phantom instructions
If I were you, I would bring the pattern with me to Joanne's or some place
like that and explain to the salespeople what happened. The people that
work there sew, so they're bound to be sympathetic, especially if you
explain that the costume is for your son. ^_^ I bet you they don't have a
problem with you taking a peek at the instructions.
Another possibility is to contact the vendor you bought the pattern from and
tell them there weren't any instructions included. They might be able to
send you some, but then again they might make you return the pattern so they
can send you another copy ...
Just a few thoughts. ^_^ Good luck with that.