<human>: We have been together for the last 4 years. When I met him he was 110kg which was a little below what I like but he has a great personality and we hit it off. Everything has been going great except for a few minor issues. He's very caring, we respect each other and have pretty similar if not identical life goals. He has also steadily gained some weight so hooray for me.

The problem is that he has always hated his weight and I hated how it made him feel. I set to aside my preferences and tried to support him by going to the gym, eating healthier with him. I've never really worried before about him actually losing the weight though because he always keeps it up for a 2-3 weeks then goes back to his old habits. However 6 months ago he switched jobs where he can walk to work (30min) and its motivated him to keep going. He's eating well, getting 1 hour of walking then he has energy to go to the gym! He's lost almost 10 kg and I don't think he's going to stop. 

He's always thought that I love his body because I love him or that I'm just being nice or love-blinded. But this is actually my preference. My father was a big sweet man and I guess I like the big cuddly body type as a result. 

I am worried that it is starting to become an issue and I'm losing my attraction to him even though he's the same sweet guy I've always known. I can't imagine my life without him but I'm just not going to be attracted to him if he keeps losing weight this drastically.

How can I break it to him without being controlling, selfish, giving him an ultimatum or seeming like I've been hiding a secret these years? 

I'm hoping there's someone who has been there who can help but any advice is really appreciated.

Can you give me the gist of the text in a nutshell?
<bot>: bf was big and I like big guys, now he's losing weight and I'm losing my attraction to him. What are my options?