It is highly unlikely that the dent is due to the old fall. As you said, the dent is visible only when he is smiling or looking up. It suggests that the dent is due to muscular activity. So a bone damage looks remote. What you can do now is that, just run your fingers gently across the forehead and see if you can feel a depression
over the bone. If you do, take him to a doctor. If not, then just relax, he is fine.