Did they do ultrasound? If he didn't get a diagnosis and sounds like he either didn't or it was misdiagnose, they need to continue doing further tests.
If he doesn't eat at all, a feeding tube or hospitalization and syringe feeding him there (if you can't syringe feed at home) will be needed until they can find the reason of anorexia.
Try different flavors and different textures. Like if he doesn't eat chunky wet food, try pate/mousse texture like Fancy Feast or vice versa. As it's been said try hand feeding too.