Insist on him getting a TIN now that it is known the SSN is invalid. Period.
For some reason IRS comes down like a ton of bricks on 1099-R payors reporting distributions with invalid SSNs, this after years of accepting payroll returns, tax withholding and W-2 filings using those same SSNs. IRS first assesses fines on the payors, and it takes a couple of years to get them waived, so they payors routinely have at least two or three pending assessments open with IRS. Basis for penalty abatement is the payors have no way knowing the SSNs were invalid, but that's not the case here.