If your father is overweight prior to the bypass, it may not be an issue, but if your father is losing mass then it may be due to lack of appropriate exercise. Depending on your weight 10 kgs could be a significant loss of muscle mass. My brother-in-law did not lose any weight of any significance following post surgery instruction.
A loss of muscle mass could be another independant issue notwithstanding bypass surgery. Age may be significant as one may not be able to exercise appropriately. Thanks for your question, take care
I lost quite a lot of weight after my bypass surgery, I assumed it was a more efficient heart enabling fluid retention to be more controlled. Saying that, when I came off the strong pain killers (about 3 months post), I started to eat a lot more and I ended up back at my original weight. If he looks OVER lean, gaunt, then I would have a Doctor check him over. Is he drinking plenty of fluids?