He did not use "American" spellings; they're all "British" (i.e., international). It's a common misconception that the -ize endings are uniquely American spelling conventions, but they're not. They are allowed and used in "British" spelling, and in a standard called Oxford spelling, they are required. That is, Oxford spelling would spell words as organize (notice the -ize), centre, favour, aeroplane, etc.

This standard is used by most international organizations because it's considered the most neutral (not to mention etymologically correct) spelling standard there is; for example, the UN, BIPM, ISO, and NATO all use it, as well as many publications/journals, such as Nature. Moreover, in most countries where English is taught as a second language, this is taught as the standard of English spelling, since the Oxford English Dictionary is considered to be the definitive source on English lexicology.

So most likely, the Bitcoin whitepaper uses Oxford spelling as well because Satoshi wanted to be as neutral and international as possible, as well as to not give away any details about his origin. Given that in other occasions/instances, he use the -ise endings in words, he's most definitely not Canadian, as Canadian spelling doesn't use this ending. He's most likely from somewhere else in the Commonwealth of Nations.

All of this, of course, assuming that he didn't try to fake all of these spellings or "slip-ups" in order to throw people off his scent.