If he was to be travelling by automobile, the Highway Patrol - State Police - Turnpike Patrol etc. who patrol the freeways may be willing to take a "watch for and check welfare" if you have some description of the vehicle, particularly if you can tell them something "distinctive" about it. A tag number is helpful, but not always necessary. (His lawyer might be able to get one for you?) An expected route would help.
The "highway guys" are generally separate agencies, and local police won't normally pass anything on to them, so you'd have to call them yourself. My past experience has been that they're a lot more helpful - if they can be - than the locals.