Casey Aldridge injured in a car accident

updated 5:18 p.m. ET April 26, 2009

Casey Aldridge, the father of Jamie Lynn Spears's 10-month-old daughter Maddie, was injured in a car accident early Sunday morning, sources tell PEOPLE.

Spears, 18, and Aldridge's sister are at Casey's side where he is hospitalized at the University of Mississippi Medical Center, according to sources. Staff at the hospital would not confirm that Aldridge is undergoing treatment there.

A family source tells PEOPLE Aldridge has "a cracked skull" and has been "in and out" of consciousness. However, the source says: "It's not as serious as we thought at first."

Aldridge, 19, was driving his truck with several passengers inside when it went off the shoulder of the road and flipped over, sources say.

"This must be just horrible for Jamie Lynn," says another source. "I know she loves him."

Police have not commented on the accident. A rep for Spears has yet to comment.