Yes, a power of attorney will enable the recipient of the power (known as the attorney in fact) to renew your registration. You can create a very narrow POA, one giving him only the power to register the car (and not, say, conduct banking for you). You should ideally let an attorney draft this for you, but if you do not want to use an attorney, you can find sample POA's online (do a "Google" search) and you can then eliminate from them any powers or authority you do not want to give your son.