rooners if you need a country could you put him in a big jumper and then he'd be Jersey 
Or green top and brown or green trousers to go as Greenland?
Or a sharks fin on his back to be Finland?
Or tie a big tie sound him to be Thailand?
Make him nod his head all day to be from the land of Nod (ok so not technically a real country - have they specified real? Or UN membership? Could be fun and interesting to do if your dc could pull it off, especially if they were happy to talk philosophically about what is a country, who decides when a country is a country (eg Scotland wanting to break free, what's happening in Ukraine and Russia right now, etc), or a nightmare otherwise...
I'm sure there must be a few others where you could think of something easy related to the name rather than do the whole national dress or proper themed thing!
That's coming from me, who realised after I'd already pushed the snooze button too many times this morning that I hadn't sorted ds1's outfit despite having been asking him for the last week what he wanted to be. Ended up wearing jeans, red top and me making a cardboard bottle with grieves marvellous medicine scrawled on it when we should have been in the car!
Will have to get Alex rider for ds1 to read so he can add that to our list of easy costumes!