Did he contribute to a 403(b) or 401(k) Plan?

EDITED to add:

The 8233 exempted him from withholding - rightly or wrongly.

Was this a temporary (one semester) hire or permanent hire?

Client may be exempt from Social Security withholding because he is covered by Canada's SS equivalent.

What kind of visa does he have?

I'd use the W-2 amounts.