"The more sex, the less they are likely to freak about the ex." Gotta disagree with this statement! Your guy is going to obsess about the ex no matter how much sex he gets from someone he doesn't care about.
Don't make the mistake of thinking you can keep him or erase the memory of the ex by having lots of sex with him. It won't make him love YOU, but you will feel used when he dumps you.
I'd back off; if he likes you, he'll be back. But make it clear you won't put up with any "ex" crap.