Rod Stewart Sued By Personal Trainer -- Your Son's a Chump!

This is America, and your medical insurance is a joke -- so says one personal trainer in Hollywood, who's now suing Rod Stewart and his son Sean ... after Sean allegedly attempted to use his med insurance to cover his physical therapy bills.

Randy Rene -- aka The Body Architect -- filed the small claims lawsuit against Sean for $5,440 for services rendered from March to May last year.

Sources close to the Stewarts tell TMZ, Sean hired Randy to help rehabilitate himself following a car accident in which he injured his lower back and shoulder.

When all was said and done, we're told Randy asked Sean for the money, but Sean told Randy to go through his medical insurance instead.

Randy insisted that wouldn't work, and now he's dragging the Stewarts to court. As for why Rod is involved, our best guess is Rod's name is on the insurance and Randy figures suing Rod too is the easiest way to get paid.

It's not the first time Sean was sued by a personal trainer -- back in 2008, some guy named Xavier sued Sean for 4 months of unpaid sessions.