"Kevin and his divorce attorney Mark Vincent Kaplan are very happy and cautiously focused on Britney's rehab follow-through," the source says. "Federline and Kaplan are not about to return custody to Britney as a reward for her going through Promises."
According to the source, Federline has taken the position that "Britney will have increased custody (after) demonstrating that she is living the life consistent with the best interest of her children."
Federline has visted her in rehab several times with their sons, most recently last weekend, and has been described as supportive of her recovery.
Britney Spears may be out of rehab, but Kevin Federline has no intention of handing over the kids any time soon.
A source close to Federline, 29, tells Usmagazine.com that the aspiring rapper and his lawyer Mark Vincent Kaplan are "ecstatic, but cautiously focused on Britney's continuing rehab recovery."
Spears entered Malibu Promises Residential Treatment Center on February 22 and was released on March 20.
Spears, 25, now has custodial visitation of 18-month-old Sean Preston and 6-month-old Jayden James, and will be granted more custody rights over her children as she proves that she can maintain a stable lifestyle.
"They are not going to return custody to Britney as a reward for doing her stint in rehab. Kevin's had the children for six weeks and he will continue to have full custody."
The source adds: "Britney has to really demonstrate she can retain sobriety. She was just introduced into a sober lifestyle. Can she maintain it?"
Source: Us Weekly