Poor guy! From this picture, it looks like his tibiotarsal-phalangeal joint is swollen. That’s the joint between the toes and longer leg bone, like an “ankle”. I recently had a duck with an infection there. The infection likely came from a bumblefoot like infection, where sometime penetrated the foot pad and entered the joint, seeding it with bacteria. Joint infections are hard to treat so my duck saw a vet, got X-rays and joint culture, then joint injection of antibiotics with oral antibiotics continued on from there. A fracture is also possible for your guy, but it would take X-rays to tell the difference. Is a vet an option?