Weight Loss (up to 12 Kg) This query!

Dear Practo Consult Team, This query regarding for my father Health issue who is of age 48 with current weight of 78 Kg (89 Kg Original) having Hypertension and Diabetes. He had unexplained weight loss of 12 Kg in 2 months where is sugar level was well below 200 in Random after eating following the normal diet. We are consulting with the Gastro Specialist Doctors to know the reason behind this drastic weight loss.Recently tested his Heart,Lungs,Liver and Kidney functions which shows normal results.Also BP is under control with regular in take of BP Medicines. Recently we checked with the Endoscopy which proves to be normal after examination.Also Colon test is planned on coming Saturday to know the any issue in Bleeding or Stools inside in the large intestine. His body shows less fluid run in the body where the skins got loosed and Tummy size is reduced almost.(Obesity Person with over weight of 89 kg) We don't know which doctor we need to consult to find his problem?