A few thoughts:
1) Check w/ Tauck Tours........they provide some of the extras like luggage handling, so you/your spouse would not be faced with having to take your own luggage to a bus or take it downstairs, etc. Even though Tauck has segments that include, when we were on their Rome, Capri, Amalfi tour, I was impressed to see that they were able to make arrangements with the hotel to transport a few of the very elderly guests in a golf-cart type vehicle (instead of their walking down a steep incline) to the tour bus. They were also very attuned to the guests' needs in the way of finding an elevator in museums instead of the very elderly guest having to take stairs. In that particular situation, it was a cancer patient with her mother who was in her late 80s. It was done without making a big deal out of it. But, you could call Tauck and see if arrangements like that could be done in certain places by the tour director.
Tauck also stays in incredibly nice hotels, great meals, etc. The are sort of the like the next step up from Rick Steves, and often better for a older guest (or anyone) that needs (or wants) a little more pampering and a higher end experience. We love to travel with them.
2) You might also want to consider a private journey tour with someone like Abercrombie and Kent, so your guide can go at a speed that is comfortable for you two and your husband won't feel like he is holding back a group in any way.
3) Or you could hire a private driver(s), combined with the train, and arrange private tour guides through our hotels. We did something like that when we were traveling in Ireland with a family member who had just finished chemo treatments a month and a half earlier. We set our own pace, and it was nice to be dropped off very, very close to each place, minimizing walking. We could go exactly where we wanted, when we wanted, stay as long as we wanted, etc. , and there was no stress of having to drive, no rental car to mess with, no filling up a fuel tank or worrying about where to park, etc. It also allowed flex time for one much-needed nap for the family member, and we could take it a little more slowly in the a.m. vs. having to meet a tour at 7:30, etc.. And, what was interesting, when I got home, I added all the expenses up, and it the price for two people was the price of what a tour for one would have cost us....and we stayed at some very high-end hotels. So, that's something you might want to consider. It give you the ultimate in flexibility.
I've been on the Heart of Italy Tour with Rick Steves (which was a fantastic trip), and while it might work for you (if you skip a lot of walking, which means you will miss a lot), I really think in this particular circumstance, another option might be best for your husband (and you).
But, definitely visit Italy. I am inspired by elderly people, who despite having to go a little more slowly, STILL GO. Sure beats not maximizing one's time on earth. Safe and happy travels!!