Kevin Federline is about to slim down.

The father of four, who was married to Britney Spears, is joining the cast of Celebrity Fit Club, a rep for VH1 confirms to DIDNTINVITEME

Also appearing on the seventh season of the show will be Federline's ex-girlfriend (and the mother of two of his kids) Shar Jackson, according to a source.

Federline, a former backup dancer, has been lambasted in the press for putting on weight since his split from Spears three years ago.

The newest season of Celebrity Fit Club premieres on VH1 Feb. 14, 2010.