Yeah, get the CRA self-employed guide, they require withholding payments usually. Essentially he does all the withholding for income tax, CPP etc. himself. Usually the US company will give him a 1042-S (which is similar to a T5) with each payment, although it's not legally required with Canadians as there is no non-resident alien withholding.
The alternative is the US company sets up a Canadian payroll and does it like a Canadian company would, but obviously they won't want to do that for one-off employees.
He'll also need to get a GST number, services rendered to foreign companies are zero-rated as they're treated as an export.
The real trick to this one imx is to get them to pay in Canadian dollars, good luck with that! Otherwise you're supposed to use the exchange rate on the day the payment was made, from
www.bankofcanada.ca or the CRA have an annual average they publish that you can use instead.
Steve.