Recently, I found that someone asked this question. The question is a good question, but it was the answer given to the question that made me think. When Answers.com was asked this question, "Who was Jotham in the Old Testament," the answer Answers.com gave was: David’s son.
Now, let us see what the Bible says. There are three Jothams mentioned in the Bible:
1. Jotham was the second of the six sons of Jahdai, a Judahite from the family of Caleb, the Hezronite (l Chronicles 2:47).
2. Jotham was the youngest of Gideon’s seventy sons and the only one who escaped the massacre of Gideon’s sons by Abimelech (Judges 9:5).
3.. Jotham was the eleventh king of Judah and the son of King Uzziah (2 Kings 15:30).
This Jotham, the son of Uzziah, was a descendant of David, but not a son of David. David did not have a son named Jotham. So, the answer provided by Answers.com is wrong.
Moral: be careful with what you read in the Internet.
Professor of Old Testament
Northern Baptist Seminary