Autistic touch-sensitivity is to do with physical properties of an object - generally texture, but heat, shape, weight etc. can also be a thing. Hence why the same autist that can't bear wearing a wool jumper (because the scratchiness is sensorily overwhelming) can be fine touching a sheepskin rug (because it's soft and non-scratchy). On the basis nickel doesn't
feel any different to any other metal, he's either genuinely allergic or munching.
Citrus "allergies" exist, but they're really oral allergen sensitivity, which is an allergy to pollen (hayfever) where the wires get crossed and the body reacts to something on the surface of the fruit. I don't think OAS is generally severe/life threatening. You can also be allergic to limonene, found in citrus fruits, but it's also found in perfumes and cleaning products and stuff, so I'm guessing it's not that.
The closest I can find for "animal fat" allergies is a page talking about how animal fats can make hayfever worse. Website look sketch as hell, so I'm not even sure that's true. You can get issues related to fat absorbtion, but these are usually trigerred by surgeries (ostomy, gallbladder removal etc.) and affect all kinds of fats, not just animal. You can also get
meat allergies, but these are to proteins in the meat in general, not just the fat, and are apparently very rare.