He owns his own work because he never signed a contract with you. When you think of businesses owning the work of their employees, that is because a lot of companies that deal with development put that clause in their contract. So under any court system, you wouldn't go anywhere with this (both stateside and in Europe). He created it and was nice enough to share it, but that is the extent of that.
I know that is really unfair, but that's life and that is how mods go. Your best action is to try to smooth things over with him and try to get his work or permission to use it. It's your only real avenue to explore (other than stealing his shit and telling him to fuck off)