Swelling in leg can be because of cardiac , renal or hepatic origin needs to be evaluated.
As we don't know anything about his medical history so difficult to suggest any thing.
Please take him to a general physician in your place for better evaluation and treatment.
He needs complete physical examination will some blood test.
Next Steps
don't keep his leg hanging while sitting.
keep his leg elevated while sleeping buy keep pillow under his legs.
low sodium diet ( salt restriction)