Most Helpful Guy
It depends on your ex. If it's that important to him, he may have backed them up somewhere, like transferring them to his computer. Most likely he hasn't, but you probably know if he's tech savvy enough to do that.
I doubt very much that you can do anything without hacking his phone directly. Do NOT succumb to blackmail of any sort. I think this is all just a macho threat.
Rule 1: Always be careful as to what leaves your phone. You never know where they will end up. Worst case is nudes with visible faces.
Most Helpful Girl
Why can't you just be honest with your boyfriend about what was going on? How would you feel if this happened with him behind your back? I feel like he should know your ex was contacting you and I don't understand why you didn't say something to him in the first place.
However I know nothing about an app that can delete texts from someone else's phone - that sounds like a huge invasion of privacy