You can claim your son on your tax return. You will have to print and mail your return to get it accepted. Sometime later in the year the IRS will request information from both you and the father to support your right to claim your son. The IRS will then decide who was right. Whoever was right will get to keep the claim on their tax return. Whoever was wrong must pay back anything they got for wrongly claiming the dependent along with penalties and interest.
Based on what you have said, you are the one who has the right to claim your son.
