I filed and my taxes were accepted. My 18 yo son is trying to do his taxes and it will not let him file with turbo tax. It keeps telling him his SS number has already been used. What do I do?

Did you already file your return and list your son on your return?  

If you did, you will have to prepare an amended return to remove your son from your return (amended return MUST be PRINTED and MAILED and can take 2-4 months to be processed.  Also, the 1040-X amended return form will not be available until mid-February.

You need to WAIT until you find out if your return has be REJECTED or ACCEPTED.
Then, it is BEST if they wait until you actually receive the refund (or payment due is taken from youyr account) based on your already filed return;  then you can start the amendment process

Above all  DO NOT DO ANYTHING until they find out if their return is rejected or accepted

For your son, there is virtually no chance that the amended return will get processed in time for him to e-file his 2019 return.

At this time he should simply PRINT and MAIL his return and not waste time this year to try to e-file

When mailing your return it is best to go to the Post Office and send it Certified with Return Receipt.