By Sara Hammel and Elizabeth Leonard
Updated July 22, 2011 03:30 PM

To all the upheaval in their lives, Maria Shriver and Arnold Schwarzenegger can now add what they call this “very scary week.”

“A few days ago our son Christopher suffered an accident at the beach,” the estranged couple say in a statement after the 13-year-old boy was hospitalized in serious condition.

Christopher was boogie boarding on Sunday in Malibu, when something went wrong and he suffered a collapsed lung, broken bones and broken ribs, a source tells PEOPLE. His mother was with him on the beach, says the source.

In the statement, his parents say, “While it has been a very scary week, Christopher is surrounded by his family and friends. He is a brave boy and is expected to make a full recovery.”

His parents also extend their gratitude to the first responders and those providing their son’s care.

“On behalf of our entire family, we want to sincerely thank the paramedics and lifeguards who responded so swiftly as well as the doctors, nurses, emergency room and hospital staff who have cared for our son,” the statement continues.

“They have been extraordinary to him and to us. We thank everyone for their thoughts and prayers and ask for respect and privacy for Christopher and our family.”