Hi Gerry and welcome!
My name is ***** ***** I will be glad to give you my thoughts on Samson's symptoms.
The lack of appetite and green diarrhea are of concern, because when a cat doesn't eat or doesn't eat properly, even for a short time, he risks developing a serious liver condition called hepatic lipidosis or 'fatty liver' and we would want to avoid this at all costs.
The diarrhea may be green because his empty stomach is producing excess acid or he may have acid reflux, which is acid coming up into his throat and burning it. This can cause nausea as well, hence, the recommendation of Pepcid.
What has me most concerned now, is that if he usually urinates a lot and is not, now, he could have a urinary tract or bladder infection, or he could have a urinary blockage, which is a medical emergency in male cats. Here is more information:
If he is overweight, at 27 pounds, he also may have developed or is on his way to developing diabetes, which will need to be treated. I'm glad you're trying to get him to lose weight via reduced caloric intake. Older cats are also more prone to develop hyperthyroidism, liver and kidney issues. Even though his thyroid tests were okay, more sensitive tests might be needed to see if this is a possibility.
At this point, I would definitely recommend an ER vet visit so Samson can be evaluated in person and additional lab work can be done; then, you will get a diagnosis and treatment to help him feel better.
I hope all will be well with your furry boy, and please let me know how he's doing and what the vet tells you. Thanks!
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