I wouldn't be surprised if you losing weight is absolutely terrifying to your ex. As long as you are obese, he can assure himself that you'll never find anyone better than he was, which seems to be important to a lot of people regardless of whether they ended the relationship or you did. It's a pride thing. But when you start losing weight, now you're probably happier with more energy, you might look more attractive, maybe dressing in nicer clothes, too... uh oh! Now he sees you were totally settling for his sorry butt and are likely to find someone way better, and then his ego will be bruised as all your friends and family tell you how lucky you are.
My ex, (who is legally not my ex yet, which is very annoying) is the sabotaging type, too. I see him 5 days a week when he picks up the kids for school or drops off from activities. I've had to tell him about the surgery as I'll need his help with the kids, and I am still on a family plan insurance so he'll need to know about deductibles and such, but I fully expect him to start panicking as the weight loss becomes noticeable. I can hardly wait to find out how he'll try to sabotage me.