Cops in Mississippi busted Casey Aldridge last week, and according to the arrest report, obtained by TMZ, he had several different prescriptions pills on him.

Officers were responding to a call for a suspicious vehicle parked alongside a road in Natchez, MS. Police say they found Casey passed out in the driver's seat and transported him to a local hospital.

Once he came to, he got the bad news ... you're getting arrested.

We're told Casey was booked for possession of Oxycodone, lorazepam, Xanax and drug paraphernalia.

Casey and Britney Spears' younger sister have one daughter together ... 10-year-old Maddie.

Jamie Lynn was 16 when she got pregnant back in 2007, and Casey was 18. They got engaged but eventually broke off their engagement.