Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you with your wee one today.
Fluid build up in the abdomen of elderly cats is never good news. It can be associated with a range of serious issues from heart disease and organ troubles to viral infections and tumors. Now if your vet couldn't make a diagnosis on their scan nor on the cellular sample, then you are caught in a wee bit of a limbo. Furthermore, if Ginger's blood protein level is low, you are also in a position where you need to take care with draining his abdomen repeatably (since you will remove what protein is in the fluid causing a further decline in the body's protein levels and as this is removed the body's imbalanced osmotic pressures will just lead to progressively quicker refilling of that belly. So, it can be a vicious cycle that worsens with each drainage).
In this case, you have two options. You can consider continuing as you are with your vet (perhaps have another sample checked to see if those illusive cells can be caught or perhaps a second scan post drainage to see if the organs and possible tumor can be seen better). Or you can request referral to a veterinary specialist facility (private or vet school). The advantage of the latter is that they will be staffed with specialists that will have a better chance of pinpointing the cause for his issues. As I am sure you can appreciate specialist veterinary ultrasonographers in private facilities will have extensive training, experience, and since they scan every day will be the best person to try and decipher what is present in Ginger's abdomen. As well, these facilities often have medical and cancer specialists that will be able to help treat him once a diagnosis is achieved.
In this case, since it sounds like you have hit a wall diagnostically and this is limiting how to continue to address Ginger's situation, a second opinion with a specialist would be a real consideration here. So, this would be something to speak to his vet about. But in the meantime, do have a word with them about his appetite. While the Mirtazipine is helping a wee bit, you may want to also have them add an anti-vomiting treatment on the alternative days as nausea often will make cats reluctant to eat. Furthermore, since the fluid will be pressing on his stomach and reducing the volume he can eat, make sure to offer small, frequent meals. And if he needs tempting, do consider a light/easily digestible diet. Examples of this would be boiled chicken, boiled white fish, scrambled eggs (made with water and not milk), meat baby food (do avoid the ones with garlic powder in the ingredients) or there are also veterinary prescription diets that can be used here (ie Hill’s I/D or Royal Canin’s sensitivity.)
Further to this, if he cannot eat much, then consider Hill's A/D (LINK) or RC Recovery from your local vet. These are both critical care diets that is comes as a soft, palatable pate. They are calorically dense, so a little goes a long way nutrition-wise and this could just help get some more calories into him even if we can’t get a huge volume of food in. As well, they can be made into a gruel for syringe feeding if required. Furthermore, if he is struggling with food, you could also use the animal version of Ensure (balanced for animals dietary requirements) called Clinicare Canine/Feline Liquid Diet (LINK). It is actually by the same people who make Ensure, but is formulated to meet out pet's dietary needs. Your vet should be able to order it for you but it is available without a prescription They also make one specifically for older cats with kidney troubles, and this could be an alternative for an older cat. This way it would a means of getting food into him and keeping his strength up while you try to identify the cause of his signs so that you can start a targeted treatment for him.
Please take care,