if i used my ex boyfriends account but it wa in his name do i have to pay it back if the bill its in his name?
You are basically saying you took money out of his wallet as you left the relationship, without his knowledge, and you're asking if that's OK. That's theft. Can you get away with it? That depends on your ex.
Going to a store and buying something knowing that you didn't intend to pay for it is fraud.
If you really need whatever it was that you purchased, you should pay for it. If you did this to stick it to your ex, return the purchases and move on.