JC - the D Day Museum was closed last March 2017 for renovation and open is in Spring. Do you know the dates? We will be there first week April but cannot find out when it re opens? Thanks
Sorry, I can't give you anymore info than the site itself. Spring 2018 is a bit ambiguous, probably for a reason and I suspect the first week of April might be stretching it. The local news site would usually announce an impending opening date but there's been nothing on there.
Even if it isn't open Portsmouth's still worth visiting, the Historic Dockyard itself will consume hours of time and I suspect your grandson will enjoy Action Stations https://www.actionstations.org. My boys, 10 and 13, love it there and a boat tour around the harbour, taking in all the naval ships and submarines as well as the history of the port should interest him (and you!).
He'll probably really enjoy the Royal Armouries Museum at Fort Nelson https://royalarmouries.org/visit-us/fort-nelson/, even though it's on the outskirts of Portsmouth and would rely on a bus or taxi to get there (a taxi from the city centre should cost around £10) it's free entry and is fantastic for military enthusiasts. It's about a 5 minute drive from where I live and my boys never tire of it, particularly exploring the tunnels.