One of the most difficult questions concerning the English language is which case of pronoun should follow "than".
Technically speaking, the nominative should probably be used. This is because the pronoun is actually the subject of an implied verb. Let's look at an example:
I was more surprised than he. (Notice "he" in the nominative as a subject pronoun).
The verb "was" is implied.
In modern speech, however, this is rarely used. "Than" is almost always followed by the accusative (the object case) of the pronoun. There is no grammatical defense for this usage unless we suppose the existence of superpronouns.
I suggest that anything following "than" in this type of sentence is actually the object and should be in the accusative. This superpronoun encapsulates the entire implied clause (subject + verb).
I was more surprised than him. (Here, "him" is a superpronoun which stands for the implied clause: "he was".)
Note that this issue is transparent for proper nouns because their accusative and nominative forms are the same and we don't notice the case.