Also, it has occurred to me that you aren't supposed to validate him. You're just supposed to be honest, caring, supportive, and loving, and your emotional response appears to be dishonest, uncaring, and hurtful, which might be easier but is in now way going to help him.
You want him to feel secure and loved? Then tell him that you aren't using him and figure out your own feelings because it's not going to help him, or your relationship, if your first reaction is to hurt him by shutting him out.
Why do you feel hurt, anyway? Does it hurt you that I'm only talking to you because it makes me feel better? That I'm using you to understand how poly works from a mono perspective?