I am sorry to hear you are worried about Bo. My name is Rebecca. I am a veterinarian and will do my best to help.
When you say he can use the bathroom, do you mean he is doing both urinating and defecating? Is his stool normal, or not?
Reluctance to jump on the couch sounds to me like he is painful somewhere, possibly his back or neck. Not eating and drinking could be signs of pain or internal illness.
Did they do any tests when you took him to your vet, such as xrays or blood tests?
If he were mine, I would be concerned about dehydration since he is not eating. I would probably try some pain medication, to see if that made him more comfortable but the safest and most effective pain medication is by prescription. The hard stomach could be pain, or he could have internal disease (liver, intestines, stomach, pancreas).
Did he have a fever when you took him to your vet?
All you can do at home is try to keep him hydrated by getting him to take water, broth, pedialyte or lick ice cubes. If you happened to have any tramadol on hand that might be safe to try for pain.
Let me know what other questions you have and how he is doing.