Their divorce was reportedly reached with an amicable agreement over the custody of their three sons, but a new report alleges that Gavin Rossdale is threatening to change the terms of the agreement with Gwen Stefani because he's fed up with how much time his sons are spending with her new beau, Blake Shelton.

Stefani is allegedly bringing her sons to Shelton's Oklahoma ranch every chance she gets, and Rossdale is reportedly not a fan of the move, and has decided to threaten to get the custody agreement changed so he can prevent her from leaving the state with Kingston, Zuma and Apollo.

"[Gavin] doesn't like the amount of time his boys are spending [at Blake's]," Hollywood Life reports. "Blake's ranch just sounds dreadful to him. He's voiced his concern to the family therapist, but he's also willing to take Gwen to court regarding their custody agreement if she continues to take the boys to Oklahoma on a regular basis."

However, since their current custody agreement, giving the pair joint legal and physical custody, doesn't have a clause which says Stefani can't take her sons out of state whenever she pleases, meaning that is Rossdale wants to force her hand about the situation, he'd have to do something about it legally.

As for why he wants to reduce the amount of time his sons spend in Oklahoma, he reportedly believes that all three boys have begun picking up bad habits since they started spending there.

"He's concerned because the boys seem to be picking up bad habits," the source said. "He's complained that the boys are going to bed with no baths, staying up late and eating junk food. Gavin knows going to court would be a huge gamble, and Gwen hasn't deprived him of custodial time [in] the agreement, but he wants to have a say about when and where the kids go when he isn't with them."

Tags: gavin rossdale Gwen Stefani Blake Shelton