why does he go back to her?

my boyfriend and i have been together for almost a year. we are engaged. he has been married before, he has a son with his ex-wife too. he always tells me how awful she treated him and how she always cheated on him, and never did anything nice to him. however...i just found out that he still paid her phone bill, and he still talks to her on a regular basis. Which i do understand that they have a child together but they dont talk about the baby...they talk about me! she knows everything about me...good, bad, funny, embarrassing. i just dont understand why if she is soooo bad! He is also a habitual liar.

should i go ahead and marry him if he is doing all of this?

Why does he talk to her about everything?