Gosh him saying "They didn't like you back then, but now they do. I was there when nobody wanted you" is super manipulative. Don't fall for that -- just see it for what it is: manipulation.
If it were me, I would split with him. And make it very clear that the split is because of his insecure behavior, not because of any weight loss. He's got to stop this jealousy & insecurity or he'll never have a successful relationship in the future.
YOU on the other hand, have a chance to be happy in life! Control what you can -- YOU. You can't control him or change him. The best help you can be to him is to tell him the way he's been acting is what has led you to break it off. He may well be a great guy and all that, but he doesn't sound like the guy for you.
I strongly encourage you to not waste any more time on a relationship that makes you unhappy. I wasted too many years with a guy who drank too much. Then it dawned on me: Love is supposed to FEEL GOOD!
And a couple of years later, I met the love of my life
best of luck to you !
I would say, "I lost weight by eating less and exercising." This is the truth. I wouldn't elaborate further. Don't lie but don't be direct. Then, I would have a talk with your dad.
