OMG you must feel physically sick because as Jill says, you just know that bird is your beautiful Sinbad. If I were you I would have sneaked on in and took him, I really would have. Is there a chance you could go back knowing they are out at the same time of day and do just that? If not then you simply have to get the police involved, you really have no option do you.
Please, please, please do not allow this man to get away with it, explain to the police that he is only after the money. Your baby will make it quite plain he is yours by the way he behaves and if it turns out he isn't then it will have been a genuine mistake and the police can't fault you for that.
Please keep us updated?