@Tinkerbell im not 100% familiar with the process but if he's wanting to keep them perhaps trying to sell the rights to the images to department stores like hobby lobby etc he could make some scratch and still retain the original
Art such as that carries an inherent copyright, so long as it can be proved/shown to have been stolen if needed. But honestly the art world sucks, I've had gigs where i did hundreds of hours of work and the commissioner fucked off l, art stolen, etc. That's why i became an art teacher, and didn't follow through with comic art