It's very poor to have post operation problems and be unable to get any treatment. In my opinion you should be a priority patient. Presumably the surgeons appointments aren't all patients with problems greater than your husbands. I'm assuming your husband is normally reasonably fit or they wouldn't have done the operation.
It's surprising the GP has no influence. If it was me I'd ring Patient Liaison and visit the hospital to find out what is going on. I'd also contact Macmillan and my MP keeping a log of all discussions and put it in writing.
It's hard for me to understand the medical problems but my simple view is your husband should cut down on liquids but not enough to dehydrate. He should be out of bed at 8am to 11pm and lie on the settee or the floor. Keep moving. Sometimes problems are caused by doing too little. Lying in bed weakens everything and might be the cause of many of the problems. It might not be though, but I'd be thinking get out of bed.