We are 20 years older than your husband, have been to Italy 17 times, and have spent 4 nights in Torino and probably a week total in Milan with a day trip to Bergamo. Most of our trips have been for a month. But our real claim to relevance to your situation is how many times one of us has traveled in Italy with various degrees of pain and ability to walk. Most recently, last fall, I needed wheelchairs in the airports, a walking boot, and could not walk more than a few hundred yards.
Our theory, which would make no sense if a trip to Italy was supposed to be a vacation or was a once-in-a-lifetime trip, is that in pain or not, difficulties or not, sick or not, we'd rather be in Italy. Even if we can't do everything we want to, or go everywhere we want to. We just transfer our life from home to Italy and go on enjoying it.
I'm trying to think what we'd do if we only had 9 nights (is that right?). We do an anti-jetlag schedule before we leave, so jetlag is not a factor for us --- is it for you? My husband can sleep on the whole flight over and I can't even nap, so 9 nights in Italy for me would be right on the edge of what would make one uncomfortable sleepless night worth it to me. Yeah, I'd probably do it.
I imagine that your husband's hip and back would still be hurting and/or stiff, so a 10-hour plane ride could get pretty uncomfortable --- perhaps a night in Milan on arrival would be helpful even though it may seem like a waste of time. It's a pretty quick train ride to Torino, though. Just leave lots and lots of extra time if walking is still slow or painful --- I would assume the need for literally slow travel while you plan now and then be pleasantly surprised if his healing is quick.
The Torino city center is flat and, depending on where you stay, quite walkable --- we stayed near the Egyptian museum, which I highly recommend (both the neighborhood and the museum). You'd want to take a taxi to the car museum and other places.
I might be inclined to skip Bergamo, or just do it as a day trip from Milan like we did. There is plenty to do in Torino and Milan. It's not that Bergamo is hilly (you take a taxi or bus to the upper city), it just might be better not to lock yourself into staying there. Simplify this trip so you can be flexible. Find places in the cities that are relaxing --- we quite like the monumental cemetery in Milan, for example, and there is a part of Torino that is more medieval (the Quadrilatero, I think) or visit its Borgo Medievale.
I wish your husband fast recovery, and a great trip (whenever you end up taking it)!