Never mind, I think I have answered my own question, as it seems I could just file the 8288-A with his return and it would generate the refund due. The only issue is the withholding agent was Canadian and doesn't have a U.S. FEIN so ProSeries won't allow an e-file. Which seems to mean I would be back to having to paper file, which is a pain.

Unless anyone has any other suggestions?

Thanks!