It's normal to sext him. so that's really nothing to worry about. Although I am 15 turning 16, i've had boys try to get in my pants and i freaked and broke up with them. It's not sad that you imgagine it, it's actually pretty normal. If your not ready for any sexual type of activity then just tell him. I think what he trying to do, is to get in your pants. But i'm jussaying what i think he's trying to do. And plus you're only 13! If your guy is the same age as you and he's proposing, then i suggest he waits as you are not near old enough to get married yet! Also if there is no communication going on when you sext or whatever then that needs to happen in order for when you are ready to have sexual type of activity. Communication is a big part of it. You need to tell each other what is right and wrong and what feels good and what dosen't. You're only young and i dare say if this one dosen't work out then there is more fish in the sea. You have a whole life ahead of you, don't waste it on some boy who wants to get in your pants. I'm not trying to sound mean either, i just know what position your in.
Hope everything works out hun :)