Isle of Wight?

There’s a pretty good bus service on the island, it doesn’t go everywhere however. Lots to do. Shanklin is gorgeous (Google image it!) Ventnor (at the bottom of the island) is a classic seaside town in a microclimate.
Lots of dinosaur history in the area, a nice little zoo and a garlic farm!

There are the model village in Godshill and the Isle of Wight Steam Railway that he might like. He also might like the chair lift to the beach at Needles.

Memory Seekers on YouTube has a playlist on the Isle of Wight that has been very useful in my planning for an upcoming trip to the Isle in September.

Bus service on the Isle of Wight is better than one usually finds on an island. Although there are exceptions, the buses tend to fan out from Newport, so you may end up there a few times, transferring. Unfortunately, Newport isn't a particularly interesting town. Look for a big tub of bus-schedule brochures at the ferry departure point. For me, having that printed schedule was a security blanket.

In addition to the places highlighted by Ade1983, Ryde and Cowes are attractive. Those are the two key ports. There are so many ferry options to the IOW it gets confusing. For your purposes, departures from Plymouth seem most convenient. My hydrofoil ran from Plymouth-Southsea to Ryde. Since I was heading to Winchester after IOW, I departed the island via the West Cowes-Southampton hydrofoil.

One of the interesting things on the island are it's Chines- deep ravine like wooded valleys. Shanklin Chine (and maybe others) have nightly illuminations when everything is lit up. I'm sure both the child and the big children would enjoy those.
@acraven meant Portsmouth, not Plymouth as the mainland port.

In some ways if going to Osborne House Southampton is the more obvious port to use on Red Funnel ferries. Although further from London it is a faster train ride then a shuttle bus to the docks. Ferries to West Cowes (then the cross river Chain ferry) or East Cowes. Easy train service Portsmouth to Southampton.

And if heading for The Needles and Alum Bay the Lymington to Yarmouth ferry is best. Lymington is like Portsmouth, a straight transfer train to ship.
Trains from London- just over 90 minutes to Brockenhurst for a quick change onto the branch shuttle. The open top bus to the Needles should still be running.
If you arrive by ferry to Ryde there is also the main line railway to Shanklin, which uses refurbished London Underground trains. Step straight off the ship onto the train.

I would imagine your grandson would enjoy both Carisbrooke Castle and Yarmouth Castle, both excellent well preserved castles to explore.

For food my children very much enjoyed The Cow. It's located on a farm and they specialise in burgers made with their own produce plus there's a variety of activities and interaction with some of the farm animals https://tapnellfarm.com/

Carisbrooke Castle also has the donkeys! These were used to turn the wheel drawing up water from the bottom of the well. There are still daily demonstrations. These are very popular so make sure you join the queue early to get a good view! It's also a good castle to explore.

Hello travel friends. Thanks for the feedback!

I am thinking we will spend 2 or 3 nights in Portsmouth instead. The dockyards sound fun, interesting and likely more appealing to our grandson. Last summer, he loved the WWII submarine on display in Chicago. Are there any tall ships in the dockyards? We are sailors and Jack Aubrey fans. And yes, I will google the area/tours.

We will probably opt for a day trip over to IOW.

So, any insights for staying/getting around in Portsmouth?

Thanks again. Love this forum.

Joe

The Hovercraft runs a shuttle bus from The Hard bus station (by the Railway Station) in connection with all flights.

Right at Gunwharf Quays is the Holiday Inn Express at which I had a good stay - good staff, standard HIE atmosphere. If you do stay there, and I'd recommend it, pack an extension cord because all electric plug points are on one side of the room, not the side with the bed. Very minor issue though...