Sources with direct knowledge tell us Tatum Thompson -- as he's now known -- was initially legally a Kardashian. Our sources say Tatum also wasn't his first name, though we're not sure what first name Khloe originally chose for the kiddo.

Eventually, our sources say Khloe decided to drop the Kardashian name for Thompson, and that's now Tatum's legal name moving forward.

As for why Khloe may have made the initial decision to leave Tristan out of things -- it seems pretty clear ... as we reported, Tristan got another woman, Maralee Nichols, pregnant after Khloe and Tristan had decided to go ahead with a surrogate to carry Tatum.

TMZ broke the story about the surrogacy in July of 2022, just a couple months after it was discovered Tristan had gotten Nichols pregnant. It was about a month later when Tatum was born, so you gotta imagine there was still a lot for Khloe and Tristan to figure out ... making it totally understandable Khloe wanted her son to have her last name.

Obviously, things have gotten better since that time, and it's almost certainly the reason Khloe decided to change Tatum's legal name to include Thompson.

As we've reported on multiple occasions, Khloe and Tristan are incredible co-parents ... hitting events together with their daughter True all the time. We were the first to report both Khloe and Tristan were at True's pre-K graduation last week ... along with exes Kylie and Travis Scott and family foe Blac Chyna.

With plenty of milestones and significant happenings for their son for the next several decades, it's nice to see Khloe and TT on the same page.