I don't know you guys of course, but I know that a lot of times a big part of a relationship is being fat together. Not only will your eating habits totally change, your body will too. He might think that you won't be attracted to him anymore when you shrink, and he might miss the times you share together with food as the center point. He could be feeling negatively judged by you--not that you're doing that--but you're judging being obese as bad (since you're getting the surgery), and therefore he might feel like you think HE is bad now. He also may not want to deal with his own issues with weight, but feels like he has to now.
If it were me, I would reassure him that though your habits might change some you will still love him no matter what. Let him know that you doing this doesn't change your opinion of him, and he doesn't need to do anything differently. Sometimes they just need some reassurance and validation, being obese is such a mental game!
