Poor core strength.

My version of this is resting my feet on a pretty tall stool under my desk at work. I've started wondering if it was related to poor core strength, even though I dont tend to think of myself has having a particularly weak core.

I think it relates to autism/ADHD and other low-muscle-tone / sensory-seeking conditions where maintaining good posture is a challenge.

Is he 21?

Situps might be one way to strengthen some core muscles.

I do that. I W-sit, too. I have hypermobile hips, and those are comfortable positions for me.

I used to sit like that and still do it with one leg while working if I am not in a meeting. I am tall and it’s just always felt more comfortable. Especially now that I have a bad back. My 8 year old does it exactly like I used to. He is also very tall and very flexible like me especially in the hips. I think we are both a bit hypermobile, but not so much that it’s a big problem. I was a D1 athlete and the flexibility was a big plus. My current back problems are likely a partial result of hypermobility and partly genetics, but nothing to do except manage it. My core is and always has been pretty strong and not worried about that at all with my son.

does he have a weak core?

I don’t think it means anything and is probably comfortable for him. I used to sit like that when at his age and didn’t think anything of it. Just make sure he wears underwear if his persists on sitting that way. One of my son’s friends sits with his feet on the chair virtually all the time and it’s rather obvious he doesn't wear them.

I’m a 6 foot tall female and have done this my whole life. I have a diagnosed hip issue which is def part of it but I have never fit in chairs properly either. Does he have a long torso?