If he still won't eat here's two formula that the Siamese Rescue recommends:

Prescription Can food: A/D mixed with KMR (consistency of pea soup) syringe in .5 cc at a time till 3 cc. Every 2-3 hours (6 hour break at night)

Or: Pedialyte mixed with Nutri-Cal.

I hope you're young one feels better soon.