How can I get him to back off?

lol well first off you never get with anyone who "just" got out of a long term relationship. i find that the other person often uses the next to continue in their mind that the relationship is still continuing. the best advice i can give you is stick it out till he finds someone else. save those text msgs and any other written contact between the two of you. if it comes down to it you might have to consider a restraining order. if words aren't getting through there's not much else you can do really. getting involved with anyone else would just seem foolish right now cause he'll probably try to sabotage any attempt at one. always think the worst of people. that way you'll be surprised when they do something good.