Thanks so much for your reply with very helpful information.
You may be looking at newly developed food allergies or IBS which is characterized by bouts of constipation and diarrhea:
IBD or colitis are also possibilities:
Sometime older cats become sensitive to the grains used as fillers in most pet foods.
Pick up any dry foods and anything with pieces, shreds or chunks. You say he does not really like them anyway.
It is possible bacteria from decaying teeth can be causing his kidney issues and his stomach to react so those should be removed. He should be on antibiotics till this is done.
Of course, nerves or stress can cause diarrhea and vomiting, however the most common cause is feeding dry food:
It is best to get him a feliway collar so the pheromone calms him wherever he is:
Most cats his age with weight loss and excessive appetite are hyperthyroid. The excess hormone burns up a lot of calories. But you say his T4 is normal. That may mean he is begining this disorder but it is not yet registering on a test.
Y/D is a food that can help:
Hills also makes food for sensitive tummies
It does sound like he is nauseated. You can give him one quarter of a 10 mg pepcid a/c every 1 2 hours:
He should also get a probiotic every day.
If you decide to do further testing the ultrasound will be more helpful (and less stressful) than x ray that would have to be done with contrast.
Feed him small frequent meals of pate foods so he does not gorge and vomit.
Please let me know what happens. I hope this has been helpful BUT if you need anything further just press reply and I will be here for you.