The 28-year-old actor shared a photo with the 32-year-old singer and 21-month-old Jagger on Friday from their visit to Lake Austin in Austin, Texas.
"#familytrip #lakeAustin loved every minute of it!!!!!so much love," he captioned the picture on Instagram.
Ross brought Jagger to watch his mom, singer Diana Ross, perform at ACL Live at The Moody Theater the night previous. He posted a video of Ross' performance online.
"Me and my baby girl watching MOMMY. You are everything MOM. Baby girl loving her Grand Mommy," the star wrote.
"Knox is such a good older brother," the singer told E! News in October 2015 after Jagger's birth. "He's always saying how beautiful she is and wanting to hold her."
"He's like a professional big brother, and he's got it down," she told AOL the same month. "He can calm her down and tell her it's okay, entertain her and dance with her."
Ross is known for playing Charlie Selby on 90210, Messalla in the Hunger Games movies and Diver Hawkes on Wicked City. He will next star in Buckout Road with Dominique Provost-Chalkley and Danny Glover.