Softwords, to be a neder he must have meant and said it in the form of a neder (e.g. konem li beiti b’Artzit HaBrit”). It might have been a shevua exceptthat that also needs spech. Writing does not count (http://www.hebrewbooks.org/pdfpager.aspx?req=14656&st=&pgnum=40). Even if it did, it might be enough to go for thirty days as that is the amount of time he would have to stay before being required to put up a mezzuzah.
Time, besides, there is a requirement to end every sentence with “eh”.