I am sorry your kitty isn't feeling well. Neoplasia refers to cancer, I am afraid. Your vet can take some survey radiographs to look for tumors, and an ultrasound can be useful for this purpose as well. Ultrasound sometimes gives a more detailed view, so it would be helpful.
There is a condition called Inflammatory Bowel Disease that may be the culprit. It is a group of gastrointestinal diseases that involves the stomach, large and small intestine or a combination thereof. The symptoms include intermittent vomiting, diarrhea (sometimes bloody), weight loss and listlessness, poor hair coat, and thickened bowel loop and enlarged intestinal lymph nodes. The treatment includes special diets, steroids (prednisone or budesonide), anti-diarrheal drugs (loperamide), antibiotics (metronidazole or tylosin), immunosuppressive drugs (azathioprene) and sometimes chemotherapeutic agents (chlorambucil). Treatment is for the life of the pet, and dosages of medications will start high and kitty will be weaned to maintenance doses. Periods of stress or excitement will cause flare ups, and meds will need to be adjusted during these times. Ultrasound and biopsy can be useful to diagnose, though your vet will most likely be ok with starting therapy without an invasive procedure like biopsy. See the following website for more information on IBD.
It sounds like your vet may be treating somewhat for IBD right now, and I hear that you are worried about your baby. I hope that you get some answers and that your kitty responds favorably to his medications and has many good years ahead of him. Please let me know if I can be of further assistance. If this has been helpful, please hit the green accept button. Best of luck with your kitty.