Cats have a life span if left indoors up until 16 years, so he has another 7 good years left.
I am a firm believer that when you take on a pet they are your responsibility for life. What are you teaching your children by getting rid of a pet because of health issues?
THAT SAID However, I understand costs that exceed what you can manage. You can surrend your cat to the Humane Society and they will give him the care he needs and find a home that doesn't mind the issues he has, hopefully.
However an older cat may or may not get adopted out, especially with health issues. Do you want your cat sitting in a cage for a long time?
I know personally, from having cats for a long time, one just died two years ago and she was 17 and had diabetes. The medication can be causing issues in the stools and you need to discuss this with your vet. Also, once he gets his diabetes manageable just like a human he can live a perfectly healthy life.
I am not laying guilt trip on you, however, keep the cat somewhat isolated if possible, keep clothes off the floor, maybe put another litter box on a level or near where he seems to be going, but try and figure it out before thinking you cannot do it. If you have had him that long it is only fair to him you do whatever you can first.
You can spend $15 on another litter box so he has it handy where he has been going, you can spend $150 on a self cleaning litter box if need be. Pets are expensive, I have three cats and a dog, so I do understand.
My suggestion is if you really cannot handle the expense, surrender him to the Humane society so he at least may get a good home.
Also you can have him tested for Toxo so if he doesn't have it you should be fine pregnant, if he has been an indoor cat his whole life and you have kids already you have probably already been exposed and fine, but please remember to talk to your OBGYN about this situation too.
Good luck.