Cannot speak for January. Went in March and the weather was great. Cool, but not COLD. Cape Horn was unpassable the day we got to it
(winds at 70 knotts) so pictures were taken at the tip and the ship went back thru Ushuaia and on.... loved the whole adventure on Celebrity Infinity. Only booked one tour from the ship to see the penguins.... the one that uses the ferry to Magdalena Island.... awesome!
Other tours we picked up at the dock in each port. Major savings compared to the ships tours. Walking is everywhere. But I did see people with wheel chairs, so I know it can be accommodated. With the smaller tour groups, your husband can choose to stay closer to the van and only stroll at his leisure.
Good luck and have a great cruise!