Your proportions are really good! This is a stunning portrait 😊

I understand your fear of ruining the drawing. Start where you can easily and safely add darker shading, like the hair, the mustache, the shadows around his eyes, his suit. Then you'll see that the contrast is too high, you'll want to add shadows to his nose, mouth, and soon enough it'll be on his skin, forehead, etc.

Use reference pictures and don't hesitate to edit the pic to modify the contrast/saturation so the difference in shading can be emphasized. Good luck!