My son's father and I have agreed that he will claim him, why is this causing an issue and not letting me file even though I have completed the correct answers?

To prevent fraudulent returns, the IRS will only accept each dependent's SSN once in their e-file system. If the other parent filed first and used your child’s SSN, no further e-files using that SSN will be accepted. 

You will need to print, sign, and mail your tax return to the IRS.  Your tax return will be accepted if you paper file.