Hi
Welcome to the forum!
Unexplained weight loss as you see in your father could be either unexplained or due to the fact that he does not eat. Since he does not explain his condition well and you are not sure whether he eats or not, it could be that he does not eat well.
Unintentional and unexplained weight loss of this magnitude is definitely not good. There are many causes like worms in stool, depression, anorexia nervosa, AIDS, cancers especially colon cancers, drug abuse, infections and loss of appetite. Loss of appetite can again be due to certain medications, drug abuse, depression, AIDS, acute and chronic infections, cancers and hypothyroidism. Either your father does not eat well or there is increased metabolism as in hyperthyroid states, HIV, cancer etc. Poor absorption as in irritable bowel syndromes and malabsorption syndromes too is possible. Malabsorption syndromes are seen in intestinal tuberculosis, HIV, tropical sprue, parasites in stool and Whipple’s Disease.
You may have looked at these options on net and it is difficult to comment without examining your father. His depression level would need to be checked, chances of bipolar disorder looked into if he has mood swings and a complete body checkup. Hope this helps. Take care!