I am sorry to hear you are worried about your cat. I am veterinarian, and will do my best to help.
This does not sound like what is seen in cats with diphacinone poisoning. Diphacinone is an anti coagulant, it stops the blood from clotting, and causes internal bleeding
. You see things like bruising on the skin, pale gums, and without the antidote (vitamin K 1, which is by prescription), they usually die. I have seen many cases of rat poisoning, and have not seen it make just one leg stiff and painful.
If you think it is the diphacinone, I would get some K1 (the vitamin K3 sold over the counter is not an antidote for rat poison unfortunately) and give him 10 mg daily for 3 to 4 weeks.
I still suspect it is something else. If he were mine, I would start with a good physical exam: listen to the heart and lungs, feel the pulses, check the color of the gums, feel the internal organs and lymph nodes, and feel the painful leg to check for infection, injury, lack of circulation to that leg, etc.
It sounds like pain medication might help him. There are very few over the counter pain relievers that are safe for cats; Tylenol for example is very poisonous to cats. You can give a cat 1/2 of a low dose aspirin, every 3 to 4 days only, but since aspirin is also an anticoagulant it might be dangerous to give to a cat that got some diphacinone. Prescription pain medication would be better and safer.
Can you get him to veterinarian, for prescription medication and an exam?
Let me know what else I can answer, or help with.