How can I convince my colleague that belief in the Quran is not valid without belief in the noble Sunnah, and respond to his skepticism regarding Ibn Kathir's interpretation of the description of Sulaiman's ant?

You must convince those who deny the Sunnah (Prophetic Tradition) of its importance from the Holy Quran, for Allah Almighty says about His Prophet (peace and blessings be upon him): "Nor does he speak of (his own) desire. It is only an Inspiration that is inspired" (Quran 53:3-4), and He says: "And We have sent down to you the Reminder that you may make clear to the people what was sent down to them" (Quran 16:44). Thus, the Prophet's (peace and blessings be upon him) clarification is a revelation and a practical application of the Book of Allah.

Furthermore, Allah Almighty says: "But no, by your Lord, they can have no faith, until they make you (O Muhammad) judge in all disputes between them, and find in themselves no resistance against your decisions, and accept (them) with full submission" (Quran 4:65), and "And whatever the Messenger gives you, take it; and whatever he forbids you from, abstain (from it)" (Quran 59:7). These verses indicate the obligation to adhere to the Sunnah of the Messenger. Whoever denies it has believed in part of the Book and disbelieved in part, and this is what Allah rebuked in His saying: "Do you then believe in a part of the Book and reject the rest?" (Quran 2:85).

The Prophet (peace and blessings be upon him) warned against sufficing with the Quran without the Sunnah, saying: "Indeed, I have been given the Book and its like with it. Verily, a time is near when a satiated man on his couch will say: 'Adhere to this Quran; what you find in it lawful, make lawful; and what you find in it unlawful, make unlawful.'"

As for Ibn Kathir's Tafsir, it is one of the most important and reliable commentaries based on narrations. The few Israelite narrations found in it, which are supported by chains of transmission, do not diminish its importance. It is in the nature of human work to have deficiencies, and this should not be a reason to turn away from the Sunnah.