Most Helpful Guy
You don't tell him. It's not up to you to decide what's right for him. If he's chosen you, it's because you have something to offer him that he values. Something he can't get just anywhere. If you think he's so wonderful, you need to get over your self loathing and just reciprocate his affections to the best of your ability and for God's sake, DO NOT TRY TO TALK HIM OUT OF LOVING YOU! That wouldn't be fair to either of you. You don't have to be societies picture of beauty for someone to find you beautiful.
Most Helpful Girl
Men generally actually like woman who resemble their mothers in some way or another. Whether looks or personality. So think about that, beauty is different for everyone... Imma girl telling you this (also I study psych)