What an awesome costume idea, never saw anything like this before. Your son got a great taste
I'm not familiar with the Ultra-Sword but it seems it's some kind of a bigger version of his normal one? According to google images at least.
If it helps you, I've got this (attachment) Kirby statue standing on my shelf here so if you need any measurements from this sword, I'm more than happy to help you but I fear that's not the right one to begin with.
One question: How big should the sword actually be? In comparision to Kirby it's roughly twice his size.
Depending on the size of the sword what I would do is basically buy some insulation foam and cut and carve/sand it to shape.
I'd cut the foam to an rectangle of the actual size of the sword and draw the outlines of it on all sides. Front, back, sides. Then start slightly carving towards the marked lines to get the overall form going.
Maybe have a wooden stick going from the handle into the blade itself giving you an actual handle and on the other hand increasidng the resistance of the foam. Once you're happy with the shape of the foam, get it covered in Bondo and sand it again to smoothen it out.
Then adding some primer and colors.
It sounds easy but there's actually some real work behind that
Edit:
http://mohrigan.tumblr.com/post/70385650947/how-to-make-a-sword-with-insulation-foam-1-copy
This shows the process I described pretty good, but there are much more detailed one's out there.
