I would recommend this tutorial http://www.cosplay.com/showthread.php?t=76828 (go through the second lot of image links).
It’s basically sandwiching corrugated cardboard together with lots of hot glue and a wooden dowel in the centre to strengthen it. You can also use foam core board instead as well.
It is actually quite hard to get the glue on the outer edges of it smooth, so what you can do is place baking paper over top and iron the glue smooth.
Or because lego deadpool’s swords look quite rounded, you can sew a sock-like cover (just trace around the sword and cut out fabric for both sides, and just sew up the edges) using shiny black spandex/thin vinyl, apply glue on the sword and slip it on. It’ll cover up bumps a lot easier than applying gesso and sanding it.
So I hope that helps and you and your son have a great time at the con! Good luck!