My dog has got an internal pus in his paw, what should i do?

Has your dog actually been seen by the vet? He needs to be on an antibiotic. If it's an abscess it may need to be lanced & drained.

You can soak the foot in a warm epsom salt mixture to help draw the infection out. Do that 3 times a day then rinse it well.
Don't wrap it or apply any ointment to it- it needs to breathe.