He was considering an Enderman this year so while researching how I would put one together, I came up with some great links so I thought I'd share..
This video is pretty detailed:
Some text tutorials I found:
Here's some more photos:
This is the only one I could find where the arms were long and skinny..
So, it looks way easier to make an Enderman costume than a Creeper..basically spray painting the boxes black and adding some cut out eyes with some purple tissue paper behind. (It'd be cool if you could light them somehow..)
Remember..if you see one this Halloween DON'T look them in the eye!!