Does Canada require a regular passport for workers going there for a few days?

Legally, Canada does not require US citizens to have a passport to enter Canada. However, because the United States requires that anyone entering the USA must have a valid passport (including US citizens) Canadian Customs Agents will quite likely deny him entry. Canada would be stuck with him until he could arrange a passport to get back home. American air carriers will not even allow him to board a flight outside the USA if he can't produce a valid passport. Tell him to get a passport.

As long as he's working for his US based firm and not seeking employment in Canada he won't need a temporary work permit.

One last point - everyone (including me) who has answered assumes that you and your son are Americans. In order to ensure you get the best responses possible you need to specify what country you're from. If you're not American the rules may be completely different.

Look up "NAFTA Visa" on wikipedia. Unless his employer has supplied a letter spelling out the terms of temporary employment and he has the correct degree as well as a passport, he will be denied entry.

It his his employer's responsibility to arrange the letters needed for a NAFTS visa.

Everyone crossing the border needs a passport.

Yes, he has to have a passport

you dont need one to get into canada but you need one to get back into the united states