Can the father sign as a TaxPreparer for my adult son’s return without his signature, although with his permission? No ... you cannot sign for the taxpayer unless you attach a POA to the return and unless you are a professional tax preparer with a PTIN number you cannot sign as the preparer.
Is the TaxPayer required to sign the return if another signs the return (on their behalf)? Either the taxpayer signs their own return OR the person with the POA signs the return.
If another can sign in place of the TaxPayer, which sections of the 1040 return are signed and what additional notes are required to accompany the other’s signature? Only the taxpayer signature line is used. POA form 2848 is used : https://www.irs.gov/forms-pubs/about-form-2848
What is the IRS source (e.g., Publication, Instruction, etc.) that supports the responses to these questions?
https://www.irs.gov/pub/irs-pdf/i1040gi.pdf on page 63
