I highly recommend that you see a tax professional (probably a tax attorney). Unless you were legally married then filing a joint tax return (or married filing separately) amounts to tax fraud. The remedy for the BF claiming a child that he was not entitled to claim was to remove the child from his tax return, not to file a false tax return. You both probably will owe back tax and probably penalties. Unmarried persons can only file as Single, or possible Head Of Household if you have a dependent child and pay more than half of maintaining the household.
You cannot amend or correct an improper joint return yourself since that requires the IRS to declare the return invalid. There is a special IRS procedure to do that that even many professionals are not familiar with since it is not common. If you do not get this legally cleared up, then it will just compound and get worse.
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**
