I think the best bet would be to get him examined by your vet. If he has sore hocks that will be why he is not running around and shifting his weight. Really they need to be checked before wrapping for infection.
My bunny Finn isn’t very active and isn’t that keen on much attention. But his not ill. Some bunnies are just like that and rather be left alone. If his eating and pooping ok then he wont be in pain, but his hocks might be sore when he moves. So do get them checked out. Try not to loose hope. It can take a while with rabbits and bringing him inside will be best. But it will take him time to get use the his new surroundings.
Try giving more food. I am still trying to build Finn up when he lost weight due to dental issues. I mush his pellets up and mix a small handful of moist rolled oats in with his pellets. Rolled oats are good for weight gain. Also try him on alfalfa hay which is more fattening.