OK, so he hates football, tennis, running, walking the dog (who really wants to be tagging along with their parents in the rain? It's actually you walking the dog, him following and observing you walk the dog), riding a bike, etc, etc, etc.
Apart from any medical things (I have PsA and EDS, so get that totally), including Vitamin D deficiency or that he needs glasses and can't see to be able to do anything physical, has it occurred to you that he just doesn't like them?
I despised all sport as a kid. I grew up in a family that only valued it - nothing I liked held any value for them. As it turns out, I just hated the things that I'd been made to do that were 'SPORT'.
I loved dance. I loved horseriding, rockclimbing and found at 16 that I really, really enjoyed using the gym (as long as it wasn't in a class or involved the elliptical or upright cycle, as they both hurt due to crappy joints/ligaments - the rowing machine is my favourite, probably because it involves sitting down, along with the weights machines - I also liked the treadmill but the PsA bolloxed that when my ankle and knee went to shit). I disliked gymnastics, but enjoy yoga/pilates. The only jumping I did outside a nice, warm dance studio was jumping the fence to cut the school 45 minute cross country run into a five minute jog out of sight of the teachers, have a nice sit down in the peace and quiet of the most wild part of the fields, then another five minute jog as the main body of the rest of the class were coming back into sight. I absolutely detest being cold and wet. But if I'm warm, dry, provided with aircon in case of getting hot and there isn't anybody else making demands of me, I'm great with physical activity.
I also buy myself decent supportive footwear, as kids' trainers were shit. A £15 pair from Primark as an adult is going to result in a nine month injury for me - but a pair for a bit more from a specialist shop, complete with gait analysis, means that I get Motion Control shoes that reduce the risk of acute or chronic injuries, make me more efficient in moving and I am definitely less tired/in pain afterwards.
That's a thought - do his feet roll in/his ankles drop? Do his shoes wear on one side? Does he have a visible arch in his foot when he's standing, or is his instep practically touching the floor? If you got him to go on tiptoes, would an arch appear?
If there are no medical/physical/nutritional/biomechanical things going on, he just doesn't like sport and fuck all is going to change that - certainly not by forcing him to do stuff. It'll just put him off even more and he won't attempt to find something he does like as he gets older.
He's artistic, you say. What about music? If he were to learn to play the drums, he ends up with an almost full body workout. It's something that on paper involves sitting down - he seems to dislike weightbearing activity particularly - it's intellectually stimulating. And it's cool - he'll never be short of offers to join bands if he can play the drums.