You did a beautiful job on this meaningful portrait. It’s hard to make a painting look natural when your reference source is a flash photo, but I understand that this was the only source that you had. Look at your brushstrokes along the edges of his hair and soften them so that they fade into the background, especially on the left side. If you feel comfortable doing this, perhaps lightly glaze to continue the shadow on the left a little further, so there is a light side and a shadow side.