Christopher and Maria Shriver
07/22/2011 at 01:45 PM EDT
Arnold Schwarzenegger and Maria Shriver's 13-year-old son is in serious condition in a Los Angeles-area hospital following an accident on a beach in Malibu.
A source close to Shriver confirms that Christopher Schwarzenegger was boogie boarding on Sunday, when something went wrong and he suffered a collapsed lung, broken bones and broken ribs. His mother was with him on the beach, says the source.
"Christopher has been in ICU since Sunday," a source tells PEOPLE. "Maria has not left his side and is sleeping at hospital."
On Friday, his older brother, Patrick, Tweeted an update
, writing: "Thank you everyone for your messages about my brother. This kid is strongest kid I ever seen. Keep praying."
And his sister, Katherine, also posted a message on Twitter: "Thank you guys for ur concern and prayers for my baby brother!" she wrote
. "He's a tough little guy and getting better! Please keep praying for him!"
Schwarzenegger has visited as well. Christopher's injuries were first reported by TMZ.com