Have you tried soaking his feet daily in a warm epsom salts bath to see if that makes a difference.
I'm not sure what's going on with him. Vitamins a couple of times a week probably won't hurt.
The swelling may be something like Gout, but it's hard to know. I would make sure he's eating well. Get a fecal float to see if he has worms, look him over for lice/mites too. While I don't think parasites are his problem, when a bird is not well, their system is not as equipped to handle a parasite load and they can decline rapidly, so staying on top of any issues like that will be helpful to his overall wellbeing.