Forgive me if I missed this, but have they tried giving Vitamin B injections??? At my previous clinic, very very commonly with older cats who were otherwise in decent health but just didn't seem to want to eat we would have come in for Vitamin B injections. They would usually perk up right away. Some needed to come in every few weeks for another injection, some didn't need more than a few rounds of it. Maybe you've mentioned this or maybe your vet has already thought about this so sorry if I missed it, just thought it warranted mentioning in case this was something you haven't tried yet