Hello, welcome to JustAnswer! I am a licensed veterinarian, and I'll be happy to help you in any way I can.
I'm sorry to hear that you've been trying to resolve this issue that Harry is having.
The signs you are describing, are consistent with a possible ulcer or other damage to the lining of the esophagus and/or the stomach.
it can be very difficult to diagnose such an issue without the use of Endoscopy, which is a procedure where a tiny camera is inserted down the throat under anesthesia.
Without such testing, the usual approach in cases of suspected ulceration, is to consider the following medicines:
Acid blockers such as famotidine, ranitidine, omeprazole, lansoprazole or others,
coating medicines such as Sucralfate,
and though it's not commonly reported in cats, treatment for Helicobacter Pylori, a bacteria that is associated with ulcers in humans and dogs, might be worthwhile as well.
at Harry's age, we also need to consider that there may be a mass or tumor in the esophagus or stomach,
leading to pain as the food or stomach acids contact the tumor.
unfortunately, cancer is common in elderly cats.
Are the drugs you mention called the same in France because this is where I live. ?
Also, he will not be put into any kind of carrier without going completely crazy. He is really feral. I am the only one who can get near him. I can cuddle him on my lap, pick him up but just the front paws. He goes crazy if you try to carry him. It's a very difficult situation as I'm sure you understand.