Listen to him when he tells you you are perfect! It sounds like this boy is looking out and he wouldn't want you to start losing weight, and he might feel it is his fault too.
Boys tend to have less body fat than girls and I've noticed that there are loads of lads in my school who are skinny so I think boys just don't start to put on weight until they get older, lucky things. Boys and girls have completely different body structures and proportion compared to girls so it's not worth comparing yourself to him.
Please try not to get too stressed out about his weight compared to yours, realtaionships are meant to make you feel good so try to enjoy it instead of stressing about weight.