Now the reason for my questions is because sudden anorexia in a cat that wants to eat and is also "yawning" is actually something we see with cats that have oral discomfort. The most common reason will be dental disease or a rotten tooth. Other potential cause that we'd also have to consider are oral ulcers (if he has any kidney issues or is a herpes virus carrier) and also growths/masses in the mouth (we can even see some cancers appear under the tongue). And just to note if he is gagging, then we'd also have to consider a possible underlying nausea here.
Now in regards ***** ***** you are right to tempt him with anything he will have. Further to what you have offered, I would note that I find they tend to be more amenable to pate style foods when their mouths are uncomfortable. As well, it can be useful to use calorically dense diets to get more nutrition in with every bite we give. Examples of these kinds of diets would be Hills A/D, Royal Canin Recovery, or even kitten wet food (since it has more fat/calories/protein). And these can all be watered down to make an easy to lap gruel or even syringe feed. Finally, in the same vein as that, there are also liquid diets like Catsure + Clinicare that could also be of use here too.
Finally, if you did think he was nauseous with this, you could potentially cover that base by treatment with an antacid. Common ones we use in cats include Pepcid (MoreInfo/Dose @http://www.petplace.com/article/drug-library/library/over-the-counter/famotidine-pepcid) and
Tagamet (More Info/Dose Here @http://www.petplace.com/article/drug-library/library/over-the-counter/cimetidine-hcl-tagamet). So, as long as he is on no other medications that you didn't mention, these could be tried to rule out nausea and see if we can get him eating.
Overall, Marvin's signs are highly suspicious of oral discomfort but we cannot fully rule out nausea. Therefore, I would suggest the above supportive care for him. But further to this, we may need the local vet to have a check of his mouth to assess it for painful rotten teeth or those other concerns. Depending on their findings, they can get him onto some cat safe pain relief +/- address the underlying cause to get him more comfortable and back to eating properly here.
Please take care,