Yes he needs to amend his return. You will have to mail your return this year so you don't have to wait for him to amend. Even if he amends it won't clear his ssn from the IRS system.
MY DEPENDENT SAID HE COULD NOT BE CLAIMED
Your dependent filed a return without saying he could be claimed on your return and your return was rejected? AFTER his return has been fully processed and he has received his refund, he has to file an amended return that says he can be claimed as a dependent on someone else’s return. Amended returns must be prepared on a Form 1040X. It takes the IRS at least four to six months to process an amended return.
Meanwhile, since your e-filed return has been rejected, you now have to print and mail it in to the IRS. You will not be able to e-file. You do not have to wait for your dependent’s amended return to be processed. Remember that when you mail a return, you need to include copies of your W-2, etc. with your return. Use a mailing service like certified mail that will track it so you will know that the IRS received it.