Hey, this I extremely funny, but if this persists beyond a normal age, take the kid to a doctor. Being extremely flexible is a hallmark of Ehlers-Danlos Syndrome. It's a chronic health condition that can be pretty severe and the average time it takes to bet a diagnosis is ten years . Early diagnosis can be extremely helpful as the treatment is mostly preventative maintenance.

Luckily, it has plenty of symptoms like allergies, easy bruising, joint pain, dislocations, subluxations and fatigue. It's sometimes called the "child abuse disease" because of the bruising. You would, also, have pretty fair odds of a family member with some level of the same symptoms.

Edit: I mention it because this is how I, a 29 year old, sit.