I would say that there is a close resemblance in several areas in the shape and layout of the map landmass but the map is an original rendering. It's like stealing a few riffs and making a new song - it happens and I think to some extent you gotta expect it. Microsoft lifted almost all their OS from Xerox and everyones lifting elements of the Apple interfaces. Some things in art too have been endlessly recycled.

I think he should own up and say "based on a map by" or even loosely based on or whatever. I mean at the end of the day he has ability and presumably could have drawn a different shape. The problem here is that he either forgot or does not think its right to attribute it. Its the wholesale cloning and passing off that I think is where its out of order as there is no injection of new talent at all. I assume in all of this that you didn't use a program to create the shape and it might be worth seeing if there are other maps he has done with same lifting of ideas and if so is there any attribution to them. Is your map CC licensed too - if it is and has SA share alike property then you have some leverage to ensure his is CC (SA) also.