There is no doubt as the the "inspiration" from your map - they are too alike for it to be a coincidence. I'm guessing he liked the shapes you got going, and took what he needed, and then changed things to separate the look of the map from yours, and perhaps fit his own campaign better.
Artist take inspiration from other artist all the time, heck how else could we come up with something "new" so often (yeah yeah, we also get inspiration from other stuff), but it is always a thin line between getting inspired and "ripping something of". For the right thing to have done would to have given you credit for inspiring him for his continent- in which case you probably would have been glad and proud instead of disapointed. Now that ship has sailed and I'm not sure what advice to give you - but I'll tell you what I might do (concidering its non-commercial) ... I'd comment on his picture on DA something about "nice to see my maps have inspired your work" or such or you could write a mail saying "I wish you would have asked..." ... or if you're non-confrontational - let it go, at least now 1 person knows it other than you
And please concider, that he probably didn't even think he did anything wrong.