Yes, separating him is a start. I feel he may have a bad sprain or possibly you are seeing an infection.
In either case, make a solution of Epsom salt and water and either wrap the leg and foot in a clean cloth or paper towel for several minutes three times a day, or hold the duck's feet and legs in a large plastic bowl or small bucket with the solution deep enough to cover the entire leg three times a day.
Examine the foot - any sign of bumblefoot? Is it warm? If so, I would lean toward thinking it is an infection, and I would slather the foot and leg with triple antibiotic ointment or cream after each e.s. soak.
If no sign of infection, I would think more likely a sprain, and if you have Traumeel or comfrey salve, slather the foot and leg with that after each soak.
Sometimes a bumblefoot infection gets into the bloodstream, and then an oral or injectable antibiotic is needed. I prefer to have a vet's guidance on that, but if that is not possible for you, most feed stores sell antibiotics.
