There are a couple of problems with travelling on a river cruise ship with a disability. If you have limited walking, and can handle stairs with railings (one flight) you may be ok. You can go on a gentle walker group, and if tiring, leave the group for a bit and meet up to get back to the ship.
Most of Europe is not set up like the US or Canada for disabilities. Toilet facilities are often up or down a flight of stairs, or a small port-a-potty type thing. Sometimes you have to climb the steps (3 sometimes high steps, but usually 3 steps) to get in and out of the busses.
Rafting is another concern, as most ships have elevators that only go to the 3 levels of cabins. If rafted against a ship that doesn't have the same set up as the one you are on, you have to climb 1 flight of steep stairs with railing up to the sundeck, cross over to the other ship, and back down a level to the lobby.
Also, as mentioned, cobbled stones are not the easiest for walking on with a walker, possible, but not always easy.
The article posted above has some really good info. I would recommend talking to a TA who has been on several lines and understands your husband's limits. It could very well be possible, and I'm sure you would enjoy it, but it's best to understand what you are going into before you shell out for a cruise.
Good luck.