Unfortunately, even though you believe your son's return was correct, if the IRS has his SSN in the system as having been claimed, there is no way around that. You have no choice but to print, sign, and mail your own return that claims him, and let the IRS sort it out later. The rejection is coming from the IRS. No way to over ride it.
When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s. Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS/state received the return.