Need to also check with California DMV. If you purchase and register the car in Colorado, no doubt you will pay sales taxes. If you purchase in CO and take the car out of state on a temporary registration, you may be able to avoid Colorado taxes.
California has a reputation for having some strict rules about taxes and title transfers, and if the final registration is to be in California, your son needs to find out how they treat sales taxes paid in another state credit, or if the mere fact of registering in CO and paying sales taxes in CO renders the issue moot for California, since it is a transfer of registration and not a sale.