Here's a link to Ypres - the ceremony at the Menin Gate is supposed to move everyone, regardless of age - and you should get other ideas of sites worth visiting from the website. It also mentions a campsite in Ypres http://battlefields1418.50megs.com/ypres_tourism.htm. Our friend is a history teacher, and regularly takes a party to the WWW1 battlefields. I'm sure you'd find plenty to interest your grandson, including visits to the trenches, war graves, etc.
As regards a beach site, near Abbeville take a look at the Camping Cheque website www.campingcheque.co.uk. La Ferme des Aulnes is a nice site, with an indoor pool in case the weather is poor, and is a short drive to the beach. There are others along the coast of Picardie and Normandie - and others may be able to recommend something.