Don't know the situation that you have so really cannot give a good comment. I know when I kiss my kids on the forehead I am staying I love you so very much cause if I kissed them on the lips that for me would feel kinds weird. I've never kissed my girl friends or wife on the forehead it would of been a light kiss on the lips or heavy depending upon the situation.
Question is...did he close his eyes or keep them open while he kissed your forehead? If he kept them open, he may think of you only as a friend, but if he closed them, depending on how long it lasted, he could have real feelings for you, and by kissing you on the forehead, he's showing you that he respects you and wants something more than a quickie. Good luck, I hope it all works out.