“Mr. Jones read a story to his son until he feel asleep.”
“Mr. Jones read a story to his son until his son fell asleep.”
Why is this? Since there are two “he“s in this sentence, it unclear to whom you are referring. The specific term for this is that you are dealing with an “ambiguous pronoun.” While it seems obvious that Mr. Jones is not the one who falls asleep, you never know—maybe Mr. Jones had a really long day at work and is exhausted!
On the other hand, “Mr. Jones read a story to his daughter until she fell asleep.” is OK because there is only one “she” in the sentence.
As a general rule, you should always keep an eye out for PRONOUNS and VERB errors on the SAT Writing because they are frequently tested.