How do I tell my parents that my ex of 7 years is willing to leave the country to be with me again?

Things aren't going to change between you an your boyfriend. He may tell you that things will be different, and you can tell yourself that, but they only thing that has changed is you are in Mexico, and he isn't. Absence tends to make both parties forget the problems in their relationship, and they tend to be a lot more forgiving. There is a reason why your parents don't like this guy. Maybe you should sit down with them and talk to them about it--as three adults. Listen to their concerns with an open mind. They have your best interests at heart and may have picked up on something that you have overlooked in the glow of your feelings for him.