I think the problem is that he already feels full, so I don't know if he could eat a day's worth of food when feeling like that. It's scary that he has gone this many days after being diagnosed. If it were my cat I would be asking how do we know when it is the last resort?You might try getting some Fortiflora probiotic powder and sprinkling that on the food. which can tempt some cats to eat.
Or try adding a little hot water, or otherwise warming up the food ( but not so that it is hot -- you don't want to create a worse food aversion by him burning his tongue!)
I will mention again that Wellness Chicken is 40 more calories per can than Hill's A/D. but if he is doing OK on the AD as far as his digestion, if you do decide to try anything different , make the change gradual, at first just mix in a little of the new.
It feels to me like the OP has already tried most ways of feeding him, and he is even avoiding syringe feeding. I would go to the tube feeding, because once he no longer needs the tube there won't be a negative association with regular eating. Right now I think he is probably wondering why his parents are trying to coax him with food while he feels full, whereas the tube feeding he will not associate with eating. He won't like it but it will go away, he will live, and IMO he will know it is a form of medication.