Tours for those with limited mobility

Venice will be a particularly difficult place for someone with mobility limitations. I would suggest having a look at the Sage Traveling website, since they specialize in HA travel. https://www.sagetraveling.com/

Yes- Venice will be difficult. There are many steps and stairs- up and down the bridges.
While there you should make use of the vaporetto as much as possible.

Rome- flatter and easier to navigate, most areas where the ancient sites are are pedestrianized and very walkable.
There are a few companies that offer golf cart tours- a Google search will bringing them up.
No experience with and not sure how close they can get you to some of the sights but could work for your family.

For Venice, I would chose your lodging very carefully. I would be looking for someplace with easy access to the vaporetto, close to the Grand Canal. That way you can cruise up and down the canal and enjoy the views. Hotel Ala comes to mind.

If your budget allows, you might consider a place ON the grand canal, where he can sit on the balcony and watch the world go by. Maybe one of the hotels near the San Zaccharia vaporetto stops. There are bridges there, but they have ramps instead of stairs.

There’s no way to see all there is to see without a lot of walking. You might consider renting a wheelchair for the trip. Benches are often few and far between. Have some honest conversations about his limitations, and expectations for the group activities. Will he be upset if the rest of the group goes off without him if he isn’t up to the activity?

You might be able to find private drivers who will take you driving by the big sites in Rome. We did this one year in Paris.

https://www.toursbylocals.com/best-of-rome-driving-tour?utm_source=google&utm_medium=cpc&device=m&network=g&matchtype=&campaignid=18181543561&devicemodel=&adgroupid=140623308917&utm_term=&gclid=Cj0KCQjwwvilBhCFARIsADvYi7K9_OEKayLmQPZgAn36vySLtF2axQ763SKt-zW6_5RUajSrOEdQGc4aAlKWEALw_wcB

What people are saying about Venice is absolutely correct. Those beautiful curved bridges actually have stairs.

I was going to suggest an Ape Calessino tour in Rome from Scooteroma https://scooteroma.com/tours/ape-calessino-tour/. But it's listed for only 3 people which wouldn't work unless y'all are really skinny. Perhaps Scooteroma could recommend a similar option for 4 adults. Or one of you could ride on the back of a Vespa like I did and you all go to the same places as mentioned toward the end of the description. I and my cranky knees found Scooteroma very accomodating and easy to work with.

This page on Venice accessible travel also gives you accessible hotels.

https://www.sagetraveling.com/Venice-Disabled-Access.

For Rome, we’ve used this golf cart tour twice.
While you won’t see the inside of sites, you’ll stop for photos at all. Great informative guides.