What Could Cause Leg Burning, Swelling, And Redness In An Obese Grandfather With Diabetes, Arthritis, And Prostate Issues?

My grandfather residing in our village is aged 78 years. He is overweight and obese. Since past couple of years he is at home. He cannot walk without support as his legs are very weak. He walks very little at home. He has diabetes, arthritis & prostrate. He has got burning sensation in the leg and swelling. Please refer pictures attached. Please suggest what made him get burning sensation in the leg, swelling & redness. What is the treatment for curing burning sensation in leg and swelling and redness.