The man in question is Tim Leissner, a senior-level businessman at Goldman Sachs.
The couple was photographed together while on vacation in St. Barts in Dec. 2013, but has kept their relationship relatively low key. The vacation was a family affair that included the lovebirds, Kimora's children and even her ex-husband.
Kimora and her ex have remained on good terms since divorcing in 2009, and they actively co-parent their two daughters. Based on his tweet, Russell seems happy to have welcomed Leissner into the family, but Kimora has yet to publicly comment on her relationship status.