My father did fasting blood sugar and it was 120 what should he do now?

Lose weight. He has pre-diabetes; to slow progression to diabetes, change lifestyle to lose 7-10% of his weight. Reading and adopting the ada diabetic diet is not a bad idea. Discuss with his pcp or endocrinologist whether metformin is right for him. Can also consider checking A1c to see of he already is a diabetic.
HGB A1C test. Your father is likely glucose intolerant. The next step is to get a hemoglobin A1c (or glycohemoglobin) test. It is a 3 month snapshot of his average glucose level. He should begin a lower carbohydrate diet, guided by his physician, of course!