This man is getting all up in my business, how can I get him to back off?

[Maria]

I was at cheer leading practice today and I passed out in the middle of practice and our coach freaked out over reacted and called my mom and my "father". Mr.Gallo (Its my "father's" name) got there first and started freaking out saying "Why are you doing this, you have a problem and you need to get better but you don't even care. Im your dad and I am worried you, you are perfect just the way yu are, bla bla bla bla bla bla." I told him to back off because he is not my father and I wanted him to leave me alone. Its not his business because he left when I was born and now that im 13 and don't want him in my life but he came back and ruined everything. When my mom got there she didn't freak out like he did because ive had anorexia for about 4 years and she is used to this tuff because things like this happen all the time. What he doesn't get is that I don't want him in my life and my health issues are MY issues and not his, but he thinks just because he has known me for like a month that he can get all up in my business. He should be taking care of his family and leave me alone. and he treated me lie if im sick or whatever and that is not true, the thing is that ive rather die skinny than live an unhappy life because im fat and that is not a big deal. Even if it was a big deal its still not his business. How can I get him to back off?