"Very well educated" and "by no means low [class]" would almost certainly mean that there were very few remaining social / regional / dialect markers in the speech. Most likely the person would speak a 'standard' English with perhaps only minor identifiers in the production of vowels. I don't think you could show this in written form. It is only the 'common folk' who have different transcriptable linguistic structures and dialect patterns. In other words just make him speak normally!