Sorry to have to welcome you to Purrs under such worrying circumstances
There's a list of hints/tips for lost cats here: http://www.purrsinourhearts.co.uk/index.php/topic,305.0.html
Not all of them may apply as Grapelli obviously won't know the area he has found himself in but worth looking through them in case there's something you haven't thought of.
A few things that come to mind:
Have the hotel offered any assistance in putting up posters on site?
Have you looked at the area on google satellite to see what is nearby and where he is likely to go (think like a cat!)?
In light of the connection that your nephew has with his cat, and his special needs, then maybe the local paper would run a little 'human interest' type article which would help spread the word?
Have you notified rescues local to the area in case he is found?
This thread below may have some useful tips in and also give you some hope - very similar circumstances, a cat escaped from her owners car at the Motorway services when the owner was on route to Germany. She was eventually found, nearby to where she had escaped, with the help of Purrs members who were local to where she had escaped. http://www.purrsinourhearts.co.uk/index.php/topic,34187.msg607650.html#msg607650
Really hope he is found safe and sound very soon