Jimmy Kimmel’s seven-year-old son has undergone his third open-heart surgery. The 56-year-old TV star has taken to social media to reveal that Billy underwent another open-heart surgery over the weekend. Jimmy – whose son was born with a congenital heart disease in 2017 – wrote on Instagram: “This weekend, our boy Billy had his third (of three, we hope) open heart surgery. We went into this experience with a lot of optimism and nearly as much fear and came out with a new valve inside a happy, healthy kid.” The comedian also thanked the “hard-working” staff at Children’s Hospital Los Angeles (C…