My Son Father Was To Suppose To File My Son But Instend He Let Somebody Else File! How Do I Find Out Who File My Baby?

Your son's father can not let any one else file his baby unless that some one else had the custody of the baby.
You can not find out who is that person. Even IRS will not disclose this.

However, while the IRS will not notify you who claimed the child as a dependent, they WILL take action against the miscreant if you tell them that only your or the child's father is allowed to claim the child as a dependent.