That Patchwork Place has an ebook available called Mosaic Picture Quilts by Pat Durbin. To make these quilts, you use a photo and place it on a grid, that you then copy with scraps of fabric. Maybe this would be a possibility for a center block. A photo of your son's bike and away you go. Another possibility is embroidery, there are loads of motorcycle patterns available online. I have embroidered BMW bikes and logos for my DH on shirts and t-shirts.