Problem being that, in order to use a W-8, you cannot be a resident alien, which it appears CanadianInHawaii is. It should be possible to get a TIN for his dependents - though it shouldn't be absolutely necessary if it's only to get short-term health insurance. It may, however, be necessary for him to buy private (i.e. not the Hawaiian state system) health insurance for only a short-term stay - and it might be necessary to get travel insurance, whereby the family would be transported back to Canada for major treatment, if they are all still considered Canadian residents and "just visiting."
Cheers,
Bev