Welcome to HCM
Your father is diabetic for more than 20 years and swelling in Left lower limb it is unilateral so possibility of infective origin is strong and treatment would take some time to give result.
function are normal it is good another possibility is obstruction to blood flow in deep veins or Lymphatics?If it is possible take opinion of a Vascular surgeon.
If he is in hospital wait for some more time and let us hope for best.
A color doppler study of Left lower limb can help.
Good luck to you