I would quilt outlines of animal motifs in the green blocks, with a thread which will show up on the fabric. You could reproduce the animals already done in embroidery, or you could quilt in new ones. Since it's for a boy, you could also quilt in masculine "toys" or activities: sports items, fishing items, bicycle, truck, the latest "monster" toy, Elmo, Big Bird, etc. Coloring books might have nice outlined images to browse through.