Wire frame sounds good. Do you have any chicken wire or hardware cloth to serve as a more secure armature? If not, I guess suspending brown paper around the frame would work. He can color the brown paper with some crayons, markers or colored pencils to get the variations in color on the wings, etc. I'm assuming he's already found a pretty good color drawing of the hawk so he can just copy that.
We have red tail hawks fly through our backyard from time to time. They sure are pretty!