since no one actually copyrighted the material, there is no grounds for copyright infringment given the lack of... well copyright to infringe.
He can't really sue you for anything for a couple reasons. Your mod is not making any financial gain and you're not selling it so the worst case is he could force you lawfully to not include it. But there can be no monetary exchange.
Also the fact he lives in Germany would make for a sticky international lawsuit, and given you guys are both poor broke kids and there is no money to be made. No one would represent you because no one stands to make any money.
In short, if you work on a mod you can't really sue anyone UNTIL the mod goes up for sale (cough dod) and you stand to get screwed over your hard work.
Cheers.