Dear friend, for singular names ending in -s, the regular genitive is usual: Davis's , Charles's . Sometimes these nouns are treated as if they were plural: Davis' , Charles' - in this case the ending -s is still pronounced even though it is absent in spelling. In your case, it is preferable to write Thomas's and finish the word with /iz/ in pronunciation.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.