Road worthiness is more about having appropriate lights, horn, seatbelts and mirrors. Engine efficiency is a big part of being road legal so as long as he didn’t alter or remove emissions equipment that will be fine. Depending on what state he lives in he might need a safety inspection but that’s definitely not even close to universal. Lastly he’d need the right paperwork like a title, registration as a hobby car, liability insurance and get the correct license plates.

If you build your own car your safety as a driver or passenger is generally not the concern of the government just that you follow the rules and make sure other drivers can see you and you can see them. Americans have completely free travel between states so there’d be no way to prevent him from traveling around and it likely would be very illegal to try to stop him if he did meet all the requirements listed.