Canada also adopted the Californian standards, so automakers will not save any R&D costs: vehicles sold in Canada will have to comply with the CARB standards. Whatever he may personally believe, the current US president has no jurisdiction in Canada.
Whatever happens, California will continue to enforce its strict vehicle emissions laws; if not directly, then by other means. Californians of both main parties remain strongly in favor of these laws, so the state agencies have a lot of scope. I could see a scheme similar to Italy's, where vehicles are excluded from urban areas based on the emissions standards they conform to. That way, anyone can buy whatever car they want, but if they ever need to go into LA or San Jose, for example, they'd better have a clean one, or be prepared to pay a hefty fine.
The thirteen states that use the stricter emissions represent 40% of all new-vehicle sales. Texas is the most notable exclusion, despite having some very serious air-quality problem zone. The electoral demographics in Texas still favor smaller, more rural communities, and they don't want to pay to help reduce air pollution in places like Dallas or Houston, but that balance is shifting as the cities grow - don't be surprised if TX starts tightening up on vehicle emissions in the next five to ten years.
What I'm saying is that there's only one way these standards are going in the long run - the wise thing would be to invest in meeting them, not try to weasel out of it.