Why does an ex boyfriend still keep in touch with his ex?

It can vary greatly,

It could be that he want's to get back together with you,

That he misses you and still values any contact he could possibly have with you,

Doesn't like the way things ended and can't live with the fact someone might dislike him and is trying to rectify that

Is just trying to mess with you

Your never going to know for sure unless you ask him specifically, in which you can then get to root of the issue