After a period of relative calm after she left rehab, Kevin Federline is reportedly worried about estranged wife Britney Spears‘ lifestyle once again.

Kevin challenged Britney for custody of their two sons during Britney’s initial post-split partying phase late last year and after a period of civil shared custody, had the boys full time (with help from Britney’s mum Lynne Spears) while the singer was in rehab in February and early March.

The pair shared custody again after she checked out of Promises Treatment Center but now that she’s partying and reportedly drinking again, Kevin is dubious about her ability to be a mother to 21-month old Sean and 9-month old Jayden.

A source close has told Us Weekly: “He thinks she’s mentally unstable and too young to be a mom.”

“[Kevin] expects full custody of his children one day.”

But he won’t abandon her completely: “He loves his kids and, to that extent, he will look after Britney.”