He said his passport will expire next year; he is going for TN renewal before then.
His concern is based on the oft-misquoted notion that one's passport must be valid for the entire period of a visa's validity. This is not universally true, and is certainly not true in the case of a TN, since no visa is being issued.
I wouldn't worry much about this. First off, we have no reports that CBP is enforcing this -- we would have heard here, no? --, and second, much of this applies to VISAS, which TN is not. --