This really cracks me up, not that you are sewing, which is cool, but that he wants to be Goku. Fortunately it won't have to fit fabulously. Actually, I'm wondering what you'll get for a wig.^^

My best effort was Super Grover, a blue furry tunic with a red flannel cape attached at the shoulders. The helmet was easy and fortunately Muppets don't really have feet. The Kid played with that costume for years.