I don't know how to help my cat anymore

In a nutshell: Sushi is 11 years old, and overweight. He has back leg issues (they're congenital, and results in very weak legs). He cannot jump, and struggles with walking. As a result, he's just been gaining more and more weight as the years go on because he can't get the exercise that a normal cat gets.

Two years ago, he stopped using the litter box. I brought it up to my vet, and she suggested some ideas, all of which worked for about a week and then he went back to not using the litter box. My fiance built him a ramp to help him get to the litter box, and while he used it for some time, he's just recently stopped again.

Well, now he has diarrhea - and it's everywhere. On the carpet, on the litter box ramp, and on him. I've been shampooing the carpet and the cat every day for a week. I called the vet, and she isn't offering me anything new - just sympathy, which doesn't help me. 

He hates the bath - what cat doesn't? - and it breaks my heart to see him so unhappy all of the time. I can't let him in the bed because of this issue. I'm out of ideas now, and I can't shake the feeling that something is wrong. Other than switching vets - which is certainly on my mind - any ideas?

That sounds almost like my Maple. Seriously, it's awful. And she's also very skittish. I've been using wipes on her but I can't keep up with her peeing (especially being at work, and now another cat has renal failure). She needs a BATH but she poops this awful stuff when she gets nervous (understatement) and picking her up would result in quite the mess.

I do feel for you. She's taken up refuge in my bedroom since July. It's been fun.

I think a second opinion is needed regarding the diahhea the vet should be helping you with that because that sounds like a medical condition. When I had this issue with my aging cat being able to step in to use the litterbox I resorted to taking plastic infant pool and cutting out the front filling it with cat litter and letting the cat just walk in and out with no climbing in. You need a little bit extra space but it worked for my kitty who got too weak in the legs.

There are definitely things you can do, like using a low edged litter box as Hexiesfriend described. Anything with a low edge will work...even a cookie sheet with 1 inch edges to hold a little litter is better than nothing.  Some people use puppy pads. I bought a Rubbermaid Storage Container and cut an opening in it for a litterbox because I wanted something that was really, really high sided, but you could use that same theory and cut an opening in any plastic or rubber container...I just duct taped around  the opening really well in case there were rough edges.     There are also diapers for cats.  Here,  check this out  http://www.handicappedpets.com/pros-and-cons-of-pet-diapers-and-belly-bands

BUT, there is a reason why your little one has diarrhea, and that needs to be addressed.  Consistent diarrhea is not normal.  If you have not recently changed foods that might be causing the issue, or done something else (changed medications, etc), then a medical exam to see what might be causing it is definitely in order.

Additionally, that extra weight is obviously not good for him, not only because of his back legs, but also it puts a strain on his heart, and makes him a greater risk for diabetes.  So, you need to see if you can try to get him to lose.  That could be a win-win situation.  We can help you with that, once you figure out the cause of the diarrhea.  I have a cat whose prone to gaining, so have personal experience with getting a cat to lose weight, as do many of use here.  It CAN be done , even if they can't get much exercise.