Dan shared the troubling news Sunday on Twitter, saying he got a call his son, Garrett, was shot in Lake Havasu, Arizona. Dan said Garrett was stable at first, but then things got worse a few hours later.

Garrett's mom, Laura, tells TMZ ... her son was walking his date to a car when he heard someone say something and then he got shot. She said he was able to call 911 himself and rushed to a local hospital ... where doctors determined the bullet hit a main blood vessel to his heart and also fractured his spine.

Laura says the injury was so bad he had to be airlifted to a Vegas trauma hospital ... and during the flight a doctor's hand was in Garrett's chest to stop the bleeding. She says the move saved his life.

We're told he's now out of surgery and was able to move his foot, which is a very positive sign.

Dan wrote ... "Thank you to the surgeon that saved my son from bleeding out on the air trip to Vegas Thank you everyone for the prayers & positive thoughts thrown our way. Garrett is out of surgery & is expected to survive."