If leg problems occurred when he was young, it may have been a hatch deformity that caused the leg to develop in such a way.
Tibial dyschondroplasia (TB), Varus-Valgus deformity, perosis, angular limb deformity, and niacin deficiency are other possible causes of leg problems in meat-breed ducks.
Most of the time with these leg conditions, as the bird ages, more weight, and stress is put on these ligaments so you often see swelling of the hock joints, and overall lethargy in the bird. Sores frequently develop on the hocks due to inactivity, which can act as gateways for bacteria to enter and can further cause conditions like septic arthritis, and osteromyleitis.
If the duck has had the condition for quite some time, without drastic measures like surgery, which may not even help, it's unlikely the problem can be resolved.
It is in the owner's opinion whether the bird should be euthanized or not. If you're seeing some of the symptoms I mentioned above, it may be best to euthanize. If you feel like you can make his way of life better, focus on administering B vitamins, specifically niacin(B3), and Omega 3's. Letting him bathe in cool water several times a day, occasionally with Epsom salts added may be benfical. When he isn't in water, center on making sure there is padding when he sits down. Slings like the one shown below can be used to prevent sores developing on the hock pads.