Sounds like you're doing the best you can at the moment, try feeding him strands of hay /grass to up his fiibre & try to help him grind his teeth down DIY style.
Pain is often one of the causes for loss of appetite, would it be possible to get a local vet to provide some Loxicom/Metacam if your usual vet could prescribe it? To save you travelling the loxicom is a dog&cat painkiller so it's likely to be easily sourced at a general vets. It's not the dearest of medications either so would help him immensely for the cost of it.
If you can syringe feed him as much as you can & as often as you can - the closer to the recommended amount the better, it'll all be a bonus.
Try to think of it as syringe feeding... Not force feeding.
Some of mine have eaten the supreme recovery off of a spoon, could you try that? Or even with his usual nuggets - pour just boiled water on them & allow them to cool to tepid /warm.
He might eat more this way if you're struggling with the syringing
Good luck & fingers crossed you can get him back on track. Look after yourself too though
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