Have you only ever tried poultry recipes for his kibble?... This may sound incredibly dumb, but has the possibility of him being allergic to chicken/poultry been discussed by the vet at all? It is a very common food allergy in dogs and cats, and the symptoms are skin irritation and vomiting.
I'm not too well versed on food allergies in hedgehogs, but I don't think it's completely unrealistic to think that poultry may very well be the culprit here if that's the only thing that has been consistent in his diet since his vomiting began. Have you tried him on any kibbles that are poultry-free, and see his reaction to that?...