My dog barks at my dad every day.?

It is just going to take time. Dad needs to make sure he never corrects him and you do. When he barks give a firm shhht noise and turn him around or into another room till dad sits. Dad should also have a stash of treats in his car or outside the door so the minute he comes in he can toss a treat to Sarge. Dad should not make a fuss when he comes in and not look at or acknowledge the behavior. Just walk in toss the treat without looking in his direction and go on about his routine. Dont sit just for Sarge or he will expect that every time.