The swelling may be associated with local inflammation due to cellulitis.
Did he had any injury or jerl over that foot . Is he having fever. Is there local rise of temperature as well over the foot
You should start with ice fomentation, keep the limb elevated over pillow, start antibiotics with analgesics, anti-inflammatory medicines.. Also, get some blood tests done like CBC, ESR,
CRP.. I will suggest you to visit nearby hospital for proper workup and management..