I agree that calling to chat with the vet is a good idea, without rushing in to action that could distress Onyx more.If I were you I'd call the vet and ask for advice. Mention to him/her your issues with Onyx and see what they have to say !!! If my cat ever had any potential problems pertaining to him and I was concerned about him, I'd call.Good luck !
Also, it the food and water out in the open or available where Onyx is sheltering? Can you offer biscuits / treats where Onyx is hiding as well as regular food, and leave water in bowl, plate, cup as options to see whether there are any preferences?
Big postive vibes in hope Onyx settles very soon.