Depends on what he's writing... if she's just a really good friend of him... not a big problem I guess.
If you are not okay with him doing that maybe you should have a talk with your boyfriend rationally saying that, him talking to his ex makes you uncomfortable and he should understand it if he's any mature.
it seems like he hasn't moved on, has he?
Do the same and see how he likes it
I agree. . It is. .. But often with children you have to use the same level of behavior so they can grasp it ... If he doesn't understand the pain he is causing then inflict the same pain so he does understand .. It's not ideal but it may be what is necessary
Dump the immature ***** already.