It's a really unsatisfying way to finish the map though, he's stuck on that tiny island with a huge army while you control 99% of the map. You've undoubtedly won at that point and just have to continually stockpile troops till you can overrun him.
Oh please, his capital has 10 gold/turn or so. It's not like he can add 10 tanks every turn. By the time you finish taking other capitals you'll have at least 150 gold/turn. Just crank out 10 balloons or bring those mainland armies back and you have more than enough to take on him.