Son originally filed claiming himself, he amended his return to not claim himself but mine still rejects because of his SSN. How do I file now and claim him as dependent?

You can still efile your return if you take an extra step.  In the past you wouldn't be able to efile after such an occurrence, but starting last year the IRS developed a workaround so the parent could still efile.  The first-listed primary taxpayer on the return (commonly a parent) can get an IP PIN and use it when efiling.  Then the parent should be able to efile.   The dependent or the secondary taxpayer (spouse) on the return do NOT have to get/use an IP PIN if they don't already have one.  Only the first-listed primary taxpayer needs to get one.

Starting last year "the IRS will accept Forms 1040, 1040-NR and 1040-SS even if a dependent has already been claimed on a previously filed return as long as the primary taxpayer on the second return includes a valid Identity Protection Personal Identification Number (IP PIN). This change will reduce the time for the agency to receive the tax return and accelerate the issuance of tax refunds for those with duplicate dependent returns. In previous years, the second tax return had to be filed by paper."

"In the scenario where the dependent has already been claimed on another tax return, the IP PIN provides an important new option. The taxpayer listed first on an e-filed tax return claiming dependents can provide their current year IP PIN when they file. If they do, the return will still be accepted. The spouse (if married filing jointly) and the dependents on the tax return don’t need to provide an IP PIN if they don’t have one."

IRS Source:

https://www.irs.gov/newsroom/irs-takes-steps-to-help-prevent-refund-delays-by-accepting-duplicate-de...

If you want to e-file claiming your son as a dependent you will have to get an Identity Protection PIN from the IRS.  Or you will have to print and mail your tax return, claiming him as your dependent.

Go to this IRS website for an IP PIN - https://www.irs.gov/identity-theft-fraud-scams/get-an-identity-protection-pin