What kind of retirement payment?  Social security, pension with a private employer, pension for civil service, or private pension?

If it's employer or private pension, was your client subject to US tax as either a resident alien or US citizen/national prior to his relocation to the US?  Did he or the employer continue to contribute to the account(s) while resident of the US?  What position was taken on those prior year returns and what basis does your client have in the account(s)?