Source: Gavin Rossdale Says If Being With Blake Shelton Makes Gwen Stefani Happy, 'So Be It'

Gavin Rossdale is taking Gwen Stefani's new relationship in stride.

One week after reps for Stefani and Blake Shelton confirmed that the pair are dating, a source close to Rossdale tells ET that the Bush frontman is focused on his family.

"Gavin is an amazing father who only cares about his kids and is determined to protect them and shield them from all of this," the source tells ET. "He is bummed that the marriage is ending, but he wants Gwen to be happy. If being with Blake makes her happy, so be it."

WATCH: Adam Levine Defends His Bromance With Blake Shelton: 'He's All Mine' -- Sorry, Gwen! 

Stefani filed for divorce from Rossdale in August, just one month before the couple would have celebrated their 13th wedding anniversary. In recent weeks, Stefani has stated that her life "blew up" in February of this year and inspired her latest single, "Used to Love You."

On Wednesday, a new report surfaced alleging that Rossdale had engaged in a three-year affair with the couple's nanny. The report claims that Stefani discovered text messages, allegedly containing nude photos of the nanny and plans to meet up for sex, between the woman and Rossdale on the family iPad.

A rep for Rossdale could not be reached by ET for comment on the report.

WATCH: Why Gavin Rossdale Is Still Wearing His Wedding Ring 

Together, Stefani and Rossdale share three sons: Kingston, 9, Zuma, 7, and Apollo, 20 months.

Shelton, who quickly and quietly split from Miranda Lambert in July after four years of marriage, seems to already be embracing Stefani's kids. When ET caught up with the country star on Tuesday night, he was all smiles discussing his new gig as host of the Nickelodeon Kids' Choice Awards. "If she wants to bring the boys out there, that's awesome!" he told ET with a big grin. See the moment in the video below.