The newborn bundle of joy was pictured wearing the cutest babygro that read "team daddy" as he watched his father compete in the US Olympic swimming trials.
Michael's fiancée and the mother of his child, Nicole Johnson, shared the snap on Instagram and gave a shout out to her husband-to-be.
Baby Boomer was born less than two months ago
"We LOVE you @m_phelps00 and we're cheering super loud for you!!" wrote Nicole.
The cheering seemed to work as Michael had the best time of the day at Tuesday's event. The 30-year-old finished first in the 200-metre butterfly event with a preliminary heat of 1 minute, 56.68 seconds.
Michael, who came out of retirement to try to qualify for the Rio games, became a father for the first time in May. He got engaged to his fiancée Nicole, a former beauty queen, in February 2015.
The adorable tot watched his dad compete in the US Olympic trials
Speaking to Today's Matt Lauer, Michael revealed that he is feeling on top of his game and couldn't be happier. It comes after the swimmer spent a stint in rehab following a DUI.
"I checked myself in because I think I was at a point in my life where something needed to change," said Michael. "I needed to figure things out."
The Olympian, who is aiming for gold at Rio, added of his training: "I want to be here, that's the difference. I had no desire to go to work out before. I want to retire how I want to retire and I have a great opportunity to do that."
Michael finished the 200-metre butterfly event in first place
The proud dad has been sharing snaps of his cute son on Instagram, and judging by the photos, Michael and Nicole are clearly relishing parenthood.
One sweet photo showed Michael feeding his son, while another was of baby Boomer in his car seat. "Finally figured this out!!!" Michael wrote.
When the swimmer marked his Father's Day, his fiancée Nicole wrote a touching caption: "You're such an incredible daddy to Boomer already & you can already see in Boomer's eyes how much he loves you!! I look forward to watching your relationship grow."