My return keeps getting rejected because of dependant ssn.

Unfortunately, this happens, but you still can claim him.  You just won't be able to do so through e-filing.  If you have other dependents, I would take that dependent off of your return for now, and file the return with the remaining dependents.  This way, you can at least get a refund through e-filing on the dependents that have not been claimed.  Then you can amend the return with the other dependent that has been claimed for the remainder of the refund.

Here's why:  when you send in the return with the dependent already claimed, the IRS has to check who has the right to claim him/her.  You will want to send in proof of that child's relationship to you (birth certificate) as well as documentation proving that he/she lived with you (school or medical records).  When the IRS sees the proof, they will issue you your refund, and will send out letters to have the other individual return the refund (or tax benefits) they received, plus applicable penalties and interest.

But it does take time (usually several months), so file what you can now electronically, and amend to get the rest.