Pregnant with newly partner but still in love with my ex of 4yrs need advice? :(

well me and my ex of 4 yrs split due to the fact of him going to prison over non payment of a fine we still spoke with letters etc but I met someone else in this time everything was fine I was quite happy in such a short amount of time I fell pregnant my ex knows im in a relationship as he sent me a letter to say from the bottom of his heart he is happy for me he hopes it works out and he treats me right and he said he will always be there for me and he hopes that one day we could get back and have a family as before he went to prison I had an ectopic pregnancy with his baby so we have been through some serious stuff.. but he is out of prison tomorrow and now all the memories and my feelings have come back for him just hit me like a gush of wind my new boyfriend knows he is out tomorrow but don't know how im feeling so if I act offish with him hes going to know something is up I don't want to be with my new partner I feel bad because im pregnant with his baby but I believe if your unhappy in a relationship it cant work this new partner tells me im a rubbish girlfriend tells me if we aint together when this baby is born he wants nothing to do with us as he wants to bring it up as a family he also said to me when we were on the verge of splitting that he wants to know what I want cos if I don't want him he will move away and get back with his ex. I don't want to be with him but hes very obsessive... I want to be with my ex but im worried to tell him im pregnant and what hes going to say I want him to understand that this pregnancy was never planned and that I could never get rid of it.. im worried he wont want nothing to do with me , and if he does what do I do about the babies father? :(