Robin Thicke is set to perform an emotional love song for estranged wife Paula Patton at the Billboard Music Awards on Sunday, May 18. Credit: Jeff Kravitz/FilmMagic

Still trying to get his girl back! Although Robin Thicke and Paula Patton announced their split this past February after eight years of marriage, the "Blurred Lines" singer, 37, has been working hard to win back his wife's affections ever since. 

His biggest gesture is yet to come—Sunday night, in fact, at the 2014 Billboard Music Awards. The R&B singer plans to impress his bride, 38, with a touching performance of a new song written for her, an insider tells Us Weekly.

"Word going around is that the song is stripped down and emotional—not quite an acoustic performance, but not showy—and he was in a good mood rehearsing it," the source tells Us. "He had lots of energy."

The chorus of the song is particularly pointed with the lyrics, "I'm gonna go get her, go get her, go get her, go get her back."

A seat has been reserved for Patton in case she decides to show up, and Thicke has kept the details quiet, the source revealed.

"He's keeping this performance under wraps and even rehearsed it in a private room this morning," the source continued. "He won't actually be trying it out on stage till the final dress rehearsal, just an hour or two before the show goes live."

The Billboard Music Awards air on ABC live from Las Vegas at 8 p.m. ET.