SC-deciding singular / plural + refferent rule

I agree with b14kumar that we have to count on the meaning of the sentence, to see to whom/which the "who" clause really refers to/ modifies.
In the first sentence, we are talking about the unique characteristic of "he" ..how it is, only "who" clause makes it clear that he "has taken Latin" ---> who clause directly modifies the subject " he" ---> the verb should be "has"

In the second one, we're talking about a "he" belonging to a group. How that group is? ..."who" clause makes it clear: a group of those who have taken Latin --> "who" clause modifies "students" ---> verb should be "have".

The sentence in this case falls into the first type --> A is better.