I'm looking to give my son a gift of a free car rental for a year.  I want to pay for the whole year plus insurance in advance with my credit card and my son can pick the car up in Washington.  Would a car rental company be a better deal than going through a car dealership?

I think you would be much better off checking with a car dealership first.  Two reasons:

1.  It will probably not be possible for you to pay yourself for your son's rental car.  The rental companies require whomever picks up the car (the renter) to show credit id (and leave a deposit) at the rental counter in their own name.  You would either have to have your son cover the costs of the rental and you would have to reimburse him  or you would have to contact the company to see if they sell gift certificates that you could order and send to him to use.... Note: they would still require him to leave his own credit card deposit.

2.  The cost of the rental car plus the rental car insurance will probably be significantly higher with a car rental company.  The price for LDW alone in Washington can add an extra $20 a day or more.  I checked as an example, the cost of a compact car with LDW from Budget for 330 days (the maximum rental length allowed) in Washington... the cost was over $20,000.

I would definitely contact a car dealer in Washington and check with them to see what options they might suggest to fulfill your request.  They may have options for a one year lease, and some insurance suggestions (ie adding your son to your own policy) for you that would be more flexible and cheaper than renting!

Hope this helps!