"She does not know that her baby is dead," the family's priest, the Rev. Don Farnan, said.
The Bresettes, a family of seven, took a weeklong spring break vacation to Destin, Fla., and were about to fly home to Overland Park, Kan., when the flight information panel fell.
Luke Bresette, the middle of five children, was killed. His brother, 5-year-old Tyler, suffered a concussion. His 8-year-old brother, Sam, had a broken leg and nose.
Tyler was released from a children's hospital Sunday; Sam was still there.
The boys' father, Ryan Bresette, and an older son and daughter, were at the airport when the sign fell but not hurt.
Heather Bresette's condition improved to serious on Sunday, University Hospital spokeswoman Nicole Wyatt said.
During their vacation, the family swam in the Gulf of Mexico and Luke went parasailing for the first time.
"His dad said he was thrilled. He was an adventurous kid. He loved sports," said Farnan, a priest at St. Thomas More in Kansas City, Mo.
After the sign fell, it took six people to lift the large board and a dozen people to hold it up while first responders administered aid. Officials were investigating how the sign fell at the newly renovated airport and took down an identical billboard on Saturday.
The renovated concourse opened March 13. It was part of an ongoing $200 million upgrade of Birmingham's airport. The construction began in June 2011 and is being overseen by Brasfield & Gorrie Global Services Group.
The Birmingham-based company said in a statement it was working with airport authorities to determine why the sign fell.
"This is a terrible tragedy that none of us fully understand, and we hope that the family who lost their loved one will find strength through prayer and the support of all of us," the statement said.
At St. Thomas More, hundreds of worshippers showed up for a Saturday morning Mass that usually has about 75 people. Luke's uncle Alex Bresette placed a Rockhurst High School jersey on the altar.
"He would have been in the Class of 2020," he told the Kansas City Star.
Ryan Bresette said in a message on Facebook that words cannot describe the pain the family feels.
In a note to his son, he wrote, "I miss and love Luke so very much. I love you Luke!"
"Ryan is especially grateful for the amazing support of the people in Birmingham. They even started a fund for the family at a bank there," Farnan said. "There are long, loving arms that stretch between Birmingham and Kansas City."